These papers and mark schemes are invaluable! They show you exactly what kinds of questions are likely to come up in the exams and the mark schemes (the answers) tell you the keywords the examiners are looking for. Download all the available past papers and mark schemes from the exam board website and use them when you are revising. This is a class example of the mantra: Practice makes perfect!

Exam Boards:

AQA: http://www.aqa.org.uk/
Edexcel: http://www.edexcel.com/
OCR: http://www.ocr.org.uk/
Scottish: http://www.sqa.org.uk/
WJEC: http://www.wjec.co.uk/