Here's a free, easy-to-use tool that can help you focus your mind when you write: the Write or Die tool from DrWicked.com.
Whenever you pause or hesitate during your writing session, this web-based tool gives you a little prod to keep you writing.
Just set the word count you want, set a time limit, and then choose your setting. In Gentle Mode, a friendly reminder message pops up whenever you stop typing. Normal Mode plays annoying show tunes to encourage you to keep writing, and Kamikaze Mode starts to delete your text if you pause for too long.
I use this tool regularly, and I've found it really helps me compose new text freely and rapidly. But most importantly, it forces me to shut off my internal editor--that's the little voice inside all writers' heads that likes to rephrase sentences, censor ideas and interfere with the creative process. For me, Kamikaze Mode seems to work best at silencing that voice--there's nothing more horrifying to me than seeing my words erased before my eyes, so I don't dare stop writing. And as a result, that little internal editor can't get a word in edgewise.
Write or Die essentially forces you to write by punishing you for pausing, and it works exceptionally well. Try it out and see what you think!