Which are (some of) your favourites GM-tips/technique ? And how do you use-them in your games ?

I love SlyFlourishs Lazy GM approach tip with the secrets (prepare n, say 10, secrets but don't define where or how the PCs will find them). Helps me think about what the session / chapter can entail and make sure the PCs get the info required to continue the story without locking me or them into specific ways to do stuff.

I start,
My most "classic tip" is never lock a clue behind a roll if they need to find the love letter hidden under the bed to find-out about the affair explaining the murder, just make sure they find-it (the failed roll can still mean they are caught searching the room)
Another of my classic one is to ask players about their character friend and foes, it helps populating a setting, you have a black smith the warrior met while serving in the army or the young ambitious political advisor the bard went in a tavern fight with and gives pretty great plot hooks So your little sister is in the school witchcraft club, and looks like they summoned something too big
For one shot, I recently experimented a lot with LARP-style black boxes in order to play a scene which occurred before the game start, as it helps giving a clear view about everyone character and their ties while keeping these scenes shorts. It's IMO a good compromise between loosing time in playing mundane life to get a feel on the character, and jumping to the action with unclear character ties/roles or expectation about normality

A few favourites from the Alexandrian:
Don't prep plots. Prep scenarios. If you give the players a goal and a world, they will make the plot themselves, and it'll be more interesting. And it's not like you wouldn't need those things for a railroad plot anyway.
Don't plan contingencies. Instead of explaining everything the party could do to get past the guard, just describe the guard. It's a lot more flexible, and it takes less time to prepare.
With the 3 clues rule, make sure to have different clue types. If all your clues are pieces of evidence, then a party who prefers to talk to people is clueless.
If you feel the need to ask "are you sure you want to do that", there might be a miscommunication to figure out. Maybe you didn't explain the situation clearly, or a player misheard you, or the player has an item to help things work out.
When creating a system within your setting (eg, nobility), add two exceptions to the neat and tidy rules. "Each region is ruled by a count, except for those over there which are ruled by comtes." This adds history to your world while making it less daunting to add more exceptions if you need them later.
- Addendum to the "Are you sure you want to do that" bullet: if a player ever does something that seems nonsensical to you, ask them what they expect to achieve by doing that. Understanding their motivation is often what resolves the miscommunication and/or allows you to steer them towards a better way to do what they're trying to do.

The players won't care about how pretty you make your maps. Make them functional and ugly, and you'll save up so much time for other prep.
- I found a map-making site that is, let's be honest, shit. The maps it makes can only ever be "good enough", and never great. This means I don't waste time trying to make them great, and can actually finish the dang things. Plus, if the players decide not to go to the noble manor, then it's no big loss.
  This idea goes for a lot of the game, actually. If you spend less time on the story, then it's no big loss if the plot takes a tangent. And they probably weren't going to be as invested in a forced narrative as they would be for something more organic.
  
  When I first started DMing as a kid, my dad told me the best thing I could do to prepare was just know the whole world. He then told me about an adventure he was running where the players, for literally no reason, started digging in the middle of a tunnel. There was a whole dungeon set up for them ready to explore, and they went 50' into the tunnel and started digging their own tunnel.
- Indeed, that's a huge one.
  Remember before streaming and VTT? You would do sketch on a white board and be happy. Some committed GM may have had a white board with a grid and copy the map from the scenario on it.
  Nice textures are nice on video games, but not on rpg
- • I refer to this as the 'Video Game Rule'. In the last thirty years the visual aspects of the hobby have become more important because we’re think we are ‘competing’ with video games. Once we realize we are making a different kind of experience it allows the story (that is the narrative elements) to outshine the graphics, if you will.-
- Also make maps that people in universe would use, not a god or modern satellite images. Romans used maps that showed main roads and villages, why would a random adventurer need a super detailed map with borders on it.

Wow, you guys are actually giving really good and useful advice.
I was about to comment “When one of my players asks whether they can do something completely unreasonable I look at them, roll a D20 openly on the table and without checking the result, say ‘no’”
But now I just feel bad :D
- I was about to comment “When one of my players asks whether they can do something completely unreasonable I look at them, roll a D20 openly on the table and without checking the result, say ‘no’”
  Actually, saying no is one of weakness, so the PC wanting to do something completly unreasonable led to some pretty great player driven session or even campaign arc.
  I just ask them how do you plan to do-it and suddenly the non reasonable plan becomes a suite of small reasonable tasks so I want the peace in the world, it's easy just give the love drug to world leader and they will all start to love each others, so first step is to put my hand in enough drug, the second is to get access to the water factory that will provide water at the next diplomatic summit, do you think the militaro industrial complex will be happy with this terrorist action ? OK that one is a bit extreme but you get the point, suddently the PC are the one writing the campaign and it's pretty cool.
  
  Sometimes that can be fun, but only if everyone at the table is onboard for a wild tangent. If the other players are bored as shit while the special snowflake starts a unicorn breeding operation, it's time to use that No. And you, the DM, are included in that too; if your players want to drag you off to write every book in the library and that's not fun for you, you have the right to say "hey maybe you should play the game I made for you instead."
- Of the responses so far, this one brought me the most joy.
- I was about to comment “When one of my players asks whether they can do something completely unreasonable I look at them, roll a D20 openly on the table and without checking the result, say ‘no’”
  Oh, GM Fiat... I always preferred the GM Camaro, but you do you... ;-)
  
  What's... What's GM camaro?

Use every opportunity to turn planning into information gathering.
I try to use every opportunity to stop the planning "phase" of the game and go to the information gathering before continuing the planning. This can be pretty much any unknown that the characters bring up, like some if -statement in their plan, some fact they are unsure about etc.
The information gathering might be anything from a simple skill check to a full adventure and after that we go right back to the planning.
This has removed a lot of planning hours that wouldn't have had anything to do with the situation they are going into.
- FWIW, I have never managed to achieve that fluently. To elegantly switch from "sitting around, planning" to interacting with the world. Retrospections are the only way that works for me
  
  Yeah, it was really a challenge to do that at first but when everyone agreed that our planning took too long and we decided to do this, it has become quite routine for us to notice when planning triggers the information gathering phase.
  And as players are getting more familiar with this, their planning has changed as well. The focus of planning is now more about coming up with relevant sources of information than trying to anticipate the future.

Read George Polti's The 36 Dramatic Situations. It's a list of plot elements that have a snappy title, a list of participants in the plot element, a brief discussion of how it works, and then (unfortunately dated) references to dramas that used them.
Using this when building a world, or a campaign, or a local setting, lets you quickly set up a bunch of conflicts (ideally with interlaced participants so that single NPCs (or PCs) can be in different roles in different dramatic situations. Then you just let the events flow logically, and as the dramatic situations get resolved you get a plot. PCs can interfere with these dramatic situations and thus have an impact on resulting plots even if the overall setting is far larger than they are.

I put my top tips here after about 15 years of collecting the best tips from hundreds of GMs:
https://slyflourish.com/top_advice.html
Let the Story Unfold at the Table
Set Up Situations and Let the Characters Navigate Them
Be On the Characters Side
Use Tools and Techniques that Help You Prepare to Improvise
Focus on your Next Game
Build Your World, Campaign, and Adventures from the Characters Outwards
Pay Attention to Pacing
Focus on the Fiction First and Mechanics Second

For depth in world-building I use a rule I call "Y-cubed". (I got it from somewhere else but can't recall the source anymore.)
For every detail you make, you ask the question "Why" three times.
So a village the characters have reached stop all work every 77 days for a festival. Why? It celebrates an ascended local hero who saved the village from a magical blight. Why 77 days? It took 77 days for effort for the blight to be defeated. ... And so on.
This is a rapid way to both build depth in your setting quickly, as well as inspire possible mysteries and intrigue for investigation later.
A slight modification works also for giving NPCs depth.

I hope it's ok that I don't put links. I think the ones that are from blogs should be easily found
Lazy GM - creates a habit of loosely planning the plot, so you can have a bag of things to use, without having to railroad, and changing the plan because of players' actions doesn't hurt
don't plan plots, plan obstacles - when you get into the habit of thinking what could be an obstacle in a situation, you don't have the game to go this or that way. You only switch between applicable obstacles
onion plots - "who needs what, what for, but they can't because of what". That way coming up with a follow up is easier
run combat like a dolphin - mainly, remember to describe things. Yes, I have to actively remember about doing that
stars and wishes - to me this is the most constructive form of after session summary. If I ask "what you didn't like?" (roses and thorns), to me it is not clear how to improve. When it's about "what you wish/wished for?" it's much easier to decide whether there was a problem with expectation management or maybe a cool idea that I passed up
yes and+no but - mainly, even if we are playing a more trad game, I don't ask for a roll if I (the plot, of course ;) ) need the thing to happen. I ask for it to answer an additional question "will the character do this well enough to uncover additional details?". Unless we are in a simulationist wounds&initiative combat, the roll to me is a plot device, not plain success/failure
And thing I came up on my own but might be only because how my mind works:
Do split the party
What I often do is present the obstacle, ask around what the characters are doing after learning that. Then I choose the sequence that I feel has the most meat on it - story to be told and go one by one. Even if an idea surprises me, I've found that by the time another player rolls their dice I already know what to do with the previous one. And when scenes have fewer participants, it's easier to manage spotlight and have lower stakes per scene

If you need to "cheat" (e.g. fudging a roll because you made your monster way too powerful) then never tell the players, it will only ruin their sense of immersion.
Try to do voices, players love them even if they're shit and it helps distinguish the NPCs. The voices don't have to be remotely good and you don't need to be good at accents, just try things like "gruff voice" for the grizzled mercenary or "weirdly enthusiastic" for the mad old wizard.

I talk only as much as is necessary to paint the scene and hurry to prompt player action.
There's an old bit of advice I read somewhere that the sooner you ask players, "What do you do?" the smoother your game is running.
Those really old AD&D modules with 3/4th the page taken up by boxed text? People tend to zone them out. WotC did studies on this and figured attention starts to drift after 2-3 sentences.
But it goes beyond boxed text. Any time the GM is sitting there talking, be it narration, exposition, or -- worst case scenario -- two NPCs having a conversation, that's time the players have to sit there trapped in an unskippable videogame cutscene.

Let them look cool.
Too much time is spent working around the players’ abilities to make a fight challenging. Some fights should look hard but have a player ability break them. Let them use the powers they earned.
This slightly ties to the idea that the game is not the players against the DM, it is the players against the world, while the DM narrates.

I recommend players make their characters together. Fate's rules for it are pretty good, and can be ported to many systems: https://fate-srd.com/fate-core/phase-trio . The whole "You all meet in a tavern for the first time" mode is a valid way to play, but I've had friends do that and then struggle with how contrived it feels to fight to the death for people they just met, or go on a whole dangerous sidequest for someone else's hobby.
I also recommend reading other systems. Not everyone needs to know dozens of games, but if you always play d20 games spending some time in a different branch of the RPG family tree can really be eye-opening. Or if you've only really played really light games, looking at how something crunchier does detail can be insightful.

This isn't from sly/Ginny/colville etc, and maybe a weird one, but for my table: "don't be afraid of a little silence". I would fear it, fill it with whatever I could, and that hurt the pacing, the quality of descriptions, and intraparty RP. A great improvement was achieved by talking less

@Ziggurat When putting a settlement on a map that you don't expect the party to go anywhere near soon, you only really need three pieces of info (beyond its location, anyway):
Name
Size (city/town/village)
Product (apples, silk, sheep, etc.) or service (government, knowledge, trade hub, etc.)
Why a product or service? It helps establish how trade happens, gives the town a reputation for the group to hear, gives you a hook from which to improvise NPCs from there, and so on.

You can storyboard ideas/set pieces with no idea how to get there. Get them written down somewhere with some extra cool details. I can guarantee now you have that list that you will see a cool option stick one of them in now and then.
your players don't have to defeat every encounter, letting your players lose (without killing them!) can be really fun. It gives them someone to hate, let's you evolve a story and makes your players think more.
embrace chaos! My players love throwing charged dust of dryness capsules at things, definitely makes fights more dynamic!
Disclaimer: I can't run a campaign without an adventure to follow. The above are really helping me go beyond the adventures as written.