Do you find yourself distracted when writing? Or perhaps the internal editor keeps looking over your shoulder?
I found this nifty little thing, called Write or Die! It's a pretty, simple but useful thing. You can either choose how long you want to write for, or how many words you want to write. Once you decide, you click "Write" and a simple text box opens. You start typing. When you stop, the screen starts turning colours. You can pick various levels of strictness, and based on that you might get some pretty annoying colours flashing or even sounds. When you finish your goal, you get a victory sound.
I wouldn't use it all the time, but sometimes when you just want to get into writing and are finding it hard to stick to, I think it may offer a productive diversion.