Write or Die
you can use it online or you can actually buy a desktop standalone version.
How it works: You set yourself a goal: X number of words in Y amount of time. It tracks the numbers of words you generate, gives you nudges if you aren't working fast enough, and lets you set "consequences." This gives you a feedback loop of a sort, and I think helps you generate bursts of writing. One thing I learned in this last nanowrimo is that writing in bursts is definitely my natural style. So, say, I set it to do 500 words in an hour (old teacher reflex! LOL). I sit there and write in its box until the goal is met. Watching that word counter helps keep me from checking my email, news, or whatever and drifting off.
When you're having trouble getting into the flow, you might try it out.