Here’s a good series on introductory improv techniques created by Jayne Entwistle for ExpertVillage. There’s a youtube playlist of it, but they were uploaded out of order, so I’ve arranged them in a way that makes them more accessible for beginners. I recommend you check out the full series.
This is the first in a series of articles on improv for screenwriters.
2. The Basic Rules
3. More on Rules
4. Yes And
6. Giving and Taking
10. Summary of rules
11. Even Advanced Improvisation is Based on the Basic Rules