I once attended a workshop with a real professional writer, and he said that writer's block is your subconscious telling you that you're going in the wrong direction. So if you're trying to write (or draw) something from a certain angle, or trying to write about/draw a particular thing, try going in the opposite direction. It's like a fly beating against a pane of glass. No matter how hard it tries, it's never going to get anywhere unless it goes in the opposite direction.
However, he also said that if you can't get anywhere no matter how hard you try, the best thing to do is to just force yourself to turn something out. It may look ugly and feel unnatural, but once you get past a certain point you get going again.
I hope some of this helps.