Here is another set of Rules that makes rounds on the internet (see our comment about Rules of Improv though). We are not aware of the source of this set of rules - if you do by all means let us know.
1) Don't Deny
Denial is the number one reason most scenes go bad. Any time you refuse an Offer made by your partner your scene will almost instantly come to a grinding halt. Example: Player A) "Hi, my name is Jim. Welcome to my store." Player B) "This isn't a store, it's an airplane. And you're not Jim, you're an antelope."
2) Don't ask open ended Questions
Open ended questions (like "Who are you?") are scene killers because they force your partner to stop whatever they are doing and come up with an answer. When you ask your partner and open ended question, you put the burden of coming up with something "interesting" on your partner - so you are no longer doing a scene together but forcing one person has to do more work than you are willing to do.
3) You don't have to be funny.
The hidden riddle of improv is that the harder you try not to be funny the more funny your scene is going to be. Why? Because it's the very best kind of improv scene you can do is an "interesting" scene, not necessarily a "funny" one. When you do an interesting scene, a very surprising thing happens… the funny comes out all by it's self.
The best ways to go are to stick to your character, stick to the story that is being told, and to stay within the reality of the scene you are playing.
4) You can look good if you make your partner look good.
When you are in a scene, the better you make your partner look the better the scene is going to be and, as a direct result, the better you are going to look. All too often, I've seen players enter a scene and I can just tell they have some really great idea about the character they are going to play or an idea they want to do. This is wonderful, but guess what? Your partner probably has absolutely no idea what's cooking in your evil little mind, and so has no idea how to react. And no matter how brilliant your idea might be, it's practically worthless if the scene as a whole goes bad.
5) Tell a story.
Storytelling is probably the easiest rule to remember but the hardest one to do. The real magic of improv is when we see the players take totally random suggestions (like a plumber and a cab driver selling shoes in a leper colony ) and somehow "make it work". If all these unrelated elements are going to come together then it's going to happen in the course of an interesting tale. So that's just what the players are going to try and do, tell us all a story.
We are what we repeatedly do. Excellence, then, is not an act, but a habit --- Aristotle