One of the most useful things I've learned about is the twenty minute writing challenge. It's simple enough- you have twenty minutes to write ____ words, and you never stop writing. For my writing class, we started off with a goal of 500 words in that time. Some people were a few words behind, some were right around 500, and some were well above it.
When you're writing anything lengthy, no matter who you are, you will get stuck. Don't think you won't- if anything, karma will strike. When you get stuck, sometimes it's only for a bit; sometimes, though, it's for a longggg time. And a good way to beat writer's block is to sit down with no distractions for twenty minutes and just write. You can pick up at a certain place, or write an excerpt. If you're writing an essay, you can get a good portion of your rough draft done. In case you aren't getting the gist of things- you can use this "challenge" on just about any literary writing. Sometimes you'll never use what you wrote; sometimes your unfiltered thoughts will make a beautiful edition to the next paper you have due. But no matter what, it gets your brain flowing!