I got an email the other day from Ben Morrow who, after apologizing for asking a strange question, asked, " how do you decide what your scenes are going to look like?"
It's actually an interesting question and seems so basic nobody ever asks it. Bear with me while I use his email as an excuse to post something, as it's been a while.
What you draw is all pretty dependent on what you've written. First you figure out where the balloons go in the panel; they have to be placed for easy reading (this can involve some rewriting). You have to keep in mind that the "tang"- the little bits hanging down to show who's speaking - can't get tangled up in knots. Clarity is important here. If the speakers switch positions from panel to panel you can use several tricks to keep continuity; like moving your "camera" around or using the action to explain the shifts (even if the action has nothing to do with the dialog- the characters are kicking a ball around or playing on the floor or something; anything to make it more visually interesting, something I had to keep in mind with a talky strip like Cul de Sac). In theater this is called blocking. It prevents characters from colliding or walking out of windows.
Then you get into stuff like close-ups, group shots, etc. Again, it's determined by your script- how can you give the words the most oomph. I always tried to include a variety of shots per strip as long as it didn't get too jumpy. Of course, repetitious shots are fun too- Petey reading on his bed with tiny variations worked well. It gives you a chance to play director, cutting from this shot to that, trying always to heighten the comedy (if that's what you're aiming for). And what you don't show is often the funniest.