I've seen the last Sherlock Holmes movie a while ago. I went with four friends, and while in the lobby, we tried to decide democratically what to see.
I wanted to see Jim Carrey's Christmas Carol, but we ended up seeing Holmes. The trailer had not impressed me, so I wasn't expecting to like it. But I did, I found it an enjoyable movie.
I've often realized how if I enter the cinema with the feeling that it's gonna suck, I end up liking it. Though at times I'm aware of this while I'm making the decision, and so I think "If I lower my expectations I'll like the movie, but then if I THINK I'm gonna like it, it means I'm probably not going to..." and so on endlessly.
You know what I think? I try to have NO expectations whatsoever. Empty my mind (which is not a hard task at all) before entering a cinema or opening a book or listening to a record for the first time.