I ran into an article on msn that is of surprising value to performers -- particularly close-up workers...the article is meant to help singles be funny on a date -- but the suggestions are very relevant to what we do. The "yes and" technique mentioned (but not fleshed out very well) is an important technique in all improv work.
http://msn.match.com/msn/article.aspx?a ... 7&menuid=6
best,
N.