Being a film nut, I always choose carefully the films I go to see, in order to avoid bad films!
I try to read one or two previews to see what the story is and who are the people involved (actors, director, writer). If there's Jim Carrey or one of my favourite actors, I go regardless of the plot or anything else.
But usually, I wait for the film to come out, read a couple of reviews and then decide to go see it or not.
The only "bad" films I saw last year were: Garfield (I loved the cartoon and couldn't help myself...and Bill Murray did the voice) and Taxi (got a free ticket). The rest were above average so apparently my technique seems to work.