Set specific goals for each revision period. Allow more time for subjects you find difficult. Check out what you do not understand. Some people find it helpful to work in groups - perhaps arranging to meet a few friends to discuss particular topics. You can use this to test each other's memory or talk through aspects you have not understood.
Some people struggle with a lack of motivation.
Improving concentration There is a separate leaflet in this series which deals specifically with improving concentration, but here are a few pointers. People vary in how long they can concentrate, so experiment and find a work pattern that suits you. If you are finding it very difficult to concentrate, start off by setting yourself a small, manageable goal. When you have achieved this, give yourself some reward. Keep repeating this goal setting and rewarding yourself.