I'm constantly proselytizing various improv techniques to LARPers

Same here. I periodically run sit-down games based entirely on long-form improv techniques, actually, and it's amazing what can happen when you ask gamers (even people with zero theatrical/improv training) to apply simple rules like "yes, and".

britgeekgrrl, you seem familiar - did you run a Cthulhu game set in a Nazi research facility back at the first WyrdCon?