Exam preparation can feel like you have a mountain to climb.
In order to make everything more manageable you have to break things down. Twenty minutes of concentrated work is far better than an hour spent twiddling your thumbs. Keep sessions short, sharp and to the point. If you ever find yourself losing concentration or momentum - take a break.
Always try to keep things fresh and interesting by changing your work location, listening to some new music or simply revising a different topic or subject. Do whatever it takes to keep concentration levels high. This way all of your time and effort will always be contributing as much as possible to the delivery of top results.