Starting in 2002, Billy Merritt (of Ninja, Robot, Pirate fame) started writing what he called “Billy Merrit’s Improv Party.” It started as a story to share his thoughts on improv and turned into a full-blown thread of awesome improv tips.

I definitely recommend reading the entire thing (though it is a bit lengthy at 50 posts). It was originally posted on the Improv Resource Center. If you haven’t checked it out, it’s probably the most active forum on improv out there.

Note: I’ve fixed some spelling and grammatical errors, but everything comes from Billy. I’ve bolded tips that particularly resonate with me.

50 Improv Tips from Billy Merritt’s Improv Party

1. Every scene has a sound track to it. All scenes have rhythm. Some scenes rock out like Rush. Some scenes hit you like the Call of the Valkries.
2. The Harold is a musical in a sense, You have the Overture, three songs that you revisit and a couple of rousing dance numbers.
3. You cannot effectivly play any GAME in any scene unless you know who you are and where you are.
4. Don’t sever your connections to the outside world, don’t become totally isolated in the community we have created, if you do, you will implode. We are conduits. We observe, take in, and record into our sense memory. We then take that information and release it on the stage. Using our improv skills we make that information dance, sing, and jump through hoops. If you stop collecting information you just have hoops.
5. It’s a lot of work only if you make it alot of work. With each line of dialog your character’s history becomes more clear, the more clear it becomes, the easier the choices become.
6. Wit is not something you just have, it is something that you must earn. You must earn it everyday [by continuing to learn].
7. Everybody has an opinion, so every character you portray should have an opinion. You start with an opinion and eventually it grows into a philosophy.
8. When in doubt talk about philosophy.
9. You become an improviser, once you feel you have it down enough that you can improvise with anyone at any time.
10. You can’t eliminate all bad habits. Sometimes you have to break the rules in order to further the scene and go where you never thought you could.
11. Don’t be afraid of the unknown, don’t play it safe. How else will you make discoveries. 
12. When a scene is started you tend to ask yourself who are these people, where are these people, and what is happening? But do you ever ask when are these people?
13. There is no heightening from blue, just more blue.
14. The most important thing is the Moment. You do all that work so that you can be in the “Moment.”
15. We need to check in with each other every now and then so that we all know what is going on, but we don’t need to do it all the time.
16. You can have a plot, but you don’t need to talk about it.
17. How can you expect to do an improvisational scene without really knowing the people in the scene. Once you know the people, the information flows all over the scene.
18. Not knowing where you are going to go in a scene yet knowing that it is going to come out all right is the core of great improv.
19. Having said all that, of course there is plot in improvisation, and most of the time it works really well. But when it works well, it is not because of the players playing to the plot. It is because of the players playing to each other and to the scene at hand.
20. Let the story come to you , don’t go looking for the story.
21. Any chance that you can place personal items into the scenes with you, do it. Make it personal, it grounds you to the scene, and it grounds you to the truth.
22. You should always walk away from an improv session and ask yourself, what have I learned, how can I use this information, how can I keep this information with me until I need it.?
23. Improvisation is an art form. Anyone can paint a picture, a good picture. But it takes more than being able to paint, to be a great artist, it takes patience, it takes observation, it takes an ability to learn when there is nothing left to learn.
24. Performing is art, it is about a sense of play, it is about growing and being allowed to fail. Producing is about business, it is about attendance, advertising, financial success. Get your art down first, develop confidence in your art , then focus on the production. Never let the production override your art. That is bad business.
25. You repeat back to your partner what you feel is important in what they just said, then both of you know whats important in the conversation you are having.
26. In acting you are told that your “being” comes from 4 places. The Head, The Heart, The Stomach, The Groin. Acting from the groin, that it is all about taking action. To find something in your scene to fuck, to engage, to become a part of. Coming from the gut, what does that mean? It means to react, to listen, to be affected. To act from the heart, is to act with emotion. To act with emotion is to feel the words that you are saying.
27. It is all about being observant, seeing things and always in the back of your head saying to yourself, I can use this in a scene.
28. To act from your head is to get in touch with your inner Robot.
29. Getting your brain programmed for “Don’t Think” takes years of preparation
30. An edit is not the end, it is the beginning of something new. 
31. The key to improvisation is patience. You will not learn everything in a year, two years, 10 years. You will never learn all there is to know, once you realize that, it becomes easier to enjoy the ride. Enjoying the ride shows patience, patience is the key.
32. Relationship and game are one in the same.
33. Your relationship is constantly defined with each exchange of dialog. Knowing your relationship defines what you will say next, the more you know the more you have to say.  Your relationship with the location will also dictate what you will do in the scene. Relationship also has to do with object work.
34. Finding the game is finding the pattern. All scenes have patterns. Patterns are structure. Structure is Game. 
35. It is important to remember to look for the first unusual thing within the reality of the scene, not the reality of the actors.
36. Words are the least important thing when it comes to communicating.
37. It is the struggle to survive that makes living so much fun. 
38. Rage is not about anger, it is about passion.  Never lose your rage, keep it inside like sushi. Eat it when you need to. Rage drives you, pirates make you alive, minjas make you take action, and The robots make sense of it all.
39. What your character believes to be true can only be heightend if the oppisite is true for someone else. Philosophies must be compared with each other so that we the audience can decide for ourselves.
40. You must constantly look for the balance in everything you do onstage, once you find the balance, then unbalance it. Create a pattern then break it. In that you will find the truth.
41. I don’t have talent, I earned talent. 
42. Success is Talent meeting Opportunity.
43. Every scene you improvise should have a Rosebud in it. Something that grounds your character into the scene, makes you take notice of your life, allows you to evaluate yourself in the place that you are at.
44. The moment you step on that stage, you own it, you are meant to be there and they were meant to watch you. That is the meaning of Stage Presence.  Stage Presence is simply the confidence to be where you are. 
45. “Humor is not jokes. It is an attitude toward being alive without which you would long ago have jumped off the 59th street bridge. Humor is not being funny. It is the coin of exchange between human beings that makes it possible for us to get through the day. Humor exists even in the humorless.” – Michael Shurtleff
46. The scene is already there before you do it. The characters have been living their lives, going to work, playing, falling in and out of love. You are just showing one moment in their lives, hopefully the funny ones. But it may not be funny to the characters at that time. You must play that real. If you play it real you will discover the humor in these peoples lives.
47. Yesing a scene does not make a scene go further, it is the “And” that breathes life into the scene.
48. There are over 100 performers that play on the UCB stage every week, of those 100 how many have given back to the space? Have painted anything? Repaired something? Lit an incense?
49. What is a moment in scenework? The moment is something that the characters, created in the scene, will remember for the rest of their lives.
50. Discovery = Truly not knowing were the scene is going to go, taking your idea and your scene partner’s idea and creating something both of you had not intended. Don’t drop your idea, meld it into another.