This delightful article has some great guidelines on being good at Improv.
The suggestions include:
- quit wiggling around so much
- keep your fingers further apart when you do object work
- stall (do object work)
- stand closer to the audience
- retain a shred of dignity (say no to inappropriate offers)
- dress normal
And because I have it at hand, this article by Will Hines about the difference between comedians and actors.