Past exam papers are a good source of revision material and can be found in many of the subject area sections of Revision World. (You can download past papers on Revision World for free, GCSE here and A-Level here).
It is vital that you look at past papers and familiarise yourself with the type and number of questions, timing, allocation of marks etc.
Use the papers and set yourself a mock. Do this systematically and seriously.
Use a quiet room with a clock; take in pens, paper and an exercise book. Sit the exam as if it were the real exam, staying within the two hour allocation and spending no more than the allotted time on each question or section.
Read and follow instructions carefully. Ask yourself: Exactly what am I being asked to do? If you are asked to summarise a passage in no more than sixty words, do not use seventy!
Decide what the question is asking you to do - e.g. whether to discuss/compare/evaluate etc - and plan what you will put into your introduction, into the paragraphs of the main body and the conclusion.
Planning ensures that you generate ideas and that you note down relevant information. If you then structure your plan clearly, you will be able to see the logical order of your ideas and you will be able to retrieve them in an appropriate sequence, thereby reducing the need to spend valuable time re-writing illogically ordered text.