The best place to begin is with an outstanding resource from OCR, an excellent teaching and learning resource, Bridging the gap between GCSE and AS/A Level Mathematics – A Student Guide (scroll down OCR’s page) or much quicker – try the new design which will get you their quicker! (Direct Link)
This is quite simply an outstanding resource for students (and their teachers!). Many exercises are provided and the answers are all at the back. OCR have very helpfully provided the document as a Word document.
With sections on Algebra, Trigonometry and graphs including examples, question practice on key topics and suggested reading before starting the A Level this will be so valuble for students.