I've never had writer's block. Not in the how-it-looks-in-the-movies sense. Sometimes I get stuck on a project and that means I've done something wrong upstream. So I go back through what I've done and read carefully and the minute I feel that something's a bit off, and go back a bit more: that's usually where the problem lies.
My general rule of thumb: if a scene isn't working, the entrance is wrong.