Coming up with a scene or sketch?
What's the best approach?
We've gone over comic characterization. So you have your characters, or let's assume you're doing more of an all-out gag approach.
Best place to start is look at your setting. Then make lists. Brainstorm things related to that subject. So if you're doing something set in a high school list out things associated with high school: Shop class, hall monitor, jocks, geeks, clubs, finals, teachers, principal, etc.
Then look at your current situation and character. You should be able to spawn off a series of jokes from there.
I'm working on a feature-length TWILIGHT spoof currently and will provide a link to it in the future. That'll be a good reference for how the lists lead to jokes.