I've been using this recently too, to try and keep me focused when writing my dissertation.

The basic gist of The Pomodoro Technique is ludicrously simple, but also extremely effective. There's apparently more to it, but I haven't felt the need to explore beyond this brilliantly basic routine:

- Write for 25 minutes without any other distractions.

- Stop for five minutes, during which time you check email, Twitter, Facebook, make tea, get up and stretch, etc.

- You have achieved one Pomodoro. Repeat until you can repeat no more.

I've found 25 minutes to be the perfect length of time. It is quick enough that I don't get bored, but also long enough to get enough done. It also encourages me to do 'just one more Pomodoro' even when I'm fed up and had enough.

If you're having trouble staying focused, I highly recommend giving it a go.

(You can use any timer, obviously, but I find using this app helpful)