I don't sit through bad movies, I either turn them off or walk out.
Saw a "special screening" for The Minus Man and walked out...
Also, the company I worked for (at the time) had a promotion associated with The Blair Witch Project 2. That was another I absolutely could not sit through.
Ironically, those are the only two movies I've seen at a "pre-screening". It made me think twice about going to see Hancock. In fact, we ended up seeing Wanted at a closer theater to avoid the crowd.
edit: exception to the above rule. Sometimes I'm in a mood to watch really bad movies. If I know they are bad going in, well, that makes it OK.