Do your best, but never try.
Just like Yoda says: "Try not. Do, or do not. There is no try." I like this quote very much in the context of improv. Always play at the top of your skills, push your boundaries, even dare to leave your comfort zone when committing to whatever it is you're doing, but never 'try'. Trying implies fear of failure, it implies some timidity.
The worst is probably when improvisers show the audience they are just 'trying' something. This perhaps is most often seen when actors engage into a song or some physical activity.