In a recent post I talked about how to revise effectively, this is essentially a follow up to that post on what you should do to stop that awful beast that is procrastination. In this day and age it is very hard to focus on one thing at once, as there are so many distractions.
I tried setting myself time limits, say an hour of revision and then 20 minutes rest, but because I would have my laptop on and music blaring I would really only get about 20 minutes worth of real work in that hour I was supposed to be revising.
So, although it might be hard to tear yourself away, you need to turn your laptop/computer off. Facebook, youtube etc are killers for procrastination. So the first thing to do is turn your laptop off and put it in a different room, you need to also do the same with your phone. If you can't safely put them in a different room, maybe hide them under your bed or any place where you can't see them when you're looking at your work.
It's better to get half an hour of solid completely focused revision rather than 3 hours where you write a couple of pages of notes while only have concentrating, you won't learn anything that way.
You should also set yourself objectives, at the moment I get one lecture done between breakfast and lunch, then one in between lunch and dinner and then two between dinner and bed that way I get 4 lectures done but it splits it up over the day, so your mind stays fresh.
So basically what I'm trying to say here is that you should revise without any distractions, and then take a break where you do no revision at all rather than going for hours whilst only half focusing on revision.
As always, good luck in this exam season :)