This is a quick list of tips for GM’s. The objective is to cover a lot of points but quickly, so as to not require a lot of reading. You might find things that are easy to understand but just never occurred to you.

This list is continuously being updated. Many points are picked out from the questions and discussion in this forum.
There’s no particular organization to the list, mostly just the order in which things were added.

If there’s something you think should be added to the list or tweaked, PM me.
Remember, the objective is brevity, to get easy points across quickly. If advice is more complicated or debatable, it probably won’t be put on the list.

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Say “Yes” More Often: If a player has an idea or desire, try to accommodate them somehow, even if you have to adjust your plans. This is generally more fun for them. (Don’t be a complete pushover though.)

Give Loot: Try to give players interesting loot for their accomplishments. Useful information or allies are good too. (Don’t give too much though.)

Everyone Matters: You might have your favorite players, but try to accommodate everyone and make everyone feel relevant. Adjust the game a little if needed. (Realize when a game just isn’t a good match for someone though.)

The Game Over a Player: While you want to try and accommodate everyone, sometimes you have to prioritize the game as a whole over a single player.

Establish a Posting Rate: For play-by-post, clearly establish an expected posting rate (once a week, once a day). People who would be too slow will know to keep out, and faster people will know to ease up. (Realize the posting rate might fluctuate.)

People Leave: People often withdraw from games, often without a word, even those you really wouldn’t expect. Be ready to move on. Don’t rely too much on anyone, player or GM.

Do You Have Time?: When starting (or joining) a game, make sure you will have enough time to commit. Think ahead a few months, don’t let moments of downtime/boredom misguide you.

Fun Over Rules: The primary goal is to have fun. If certain rules are getting in the way of that (tedious, constantly looking things up), ignore or adjust them.

Common Sense Over Rules: Generally when using a system (DnD, GURPS, etc.) you’ll follow the rules by the book. However, in certain situations, certain rules just might not make any logical sense. In these cases, ignore or adjust the rules.

Don’t Get Too Attached to Your NPC’s: Be willing to let your awesome bad guy die earlier than expected if the players catch you by surprise, otherwise the players may get frustrated that their actions don't really matter. Don’t put the spotlight too much on one of your NPC’s, it should be more focused on the player characters.

Admit Your Limitations and Mistakes: If you are inexperienced with a certain rulebook (or maybe GM’ing in general), let your players know as much. They’ll be more willing to forgive you if/when you make mistakes. (Don’t be a complete pushover though, players giving advice might not be as expert as they think they are.)
Similarly, admit when you’ve made a mistake, and try to correct it.