Regular breaks, and vary the topic that you're studying if you can so that you're not spending too long on the same thing. Try to minimise distractions by making sure the room is nice and warm, and quiet. Make sure you have lots of space to spread things out as you need to and that you're comfortable. Also make sure that you've got plenty of light. Make yourself a drink and get some fruit / other snack, so that you don't have an excuse to get up and go and get some! Then set yourself a time during which you'll revise, say 45 minutes, and then reward yourself with a break afterwards. Don't try to work at it for too long in one go as that will make your concentration go down; in the long run, taking regular breaks is more productive. If it works for you, then try to set yourself targets - but make sure that they're realistic otherwise this can get demoralising. So decide that in the next 45 minute stretch you are going to revise particular topic X, and then after your break, move onto a different subject. Sometimes varying the revision by attempting past papers / questions can help. If you have foreign language exams then sometimes revising in pairs can help as you can talk to each other and ask each other questions etc. One thing that used to motivate me sometimes was buying myself some new pens and paper! Some funky coloured pens or something, and a nice new notebook, and then there's motivation to sit down and use them and do work

If you're finding it's your environment that's distracting you, then maybe try to find somewhere else to work - one year I decided to work in the library, and I was strict with myself and myself stay in there 9-5 (with breaks) and do work, but then I could go home and spend the whole evening relaxing - and that was useful because it made the distinction between 'work' and 'relaxing'. There's less distraction in the library but the downside is you can't have food/drink, and taking breaks isn't as easy because htere's not so much to do. Good luck for your exams, is it GCSEs / A-levels that you're doing?