Make sure when you are doing past papers that you are doing them efficiently. I find it very useful to have the following approach when doing a paper:
1) Sit the paper in exam conditions under the usual time constraint
2) After each paper, mark it then anything you got wrong go over that topic in your book making sure you understand it before you move on, this will take a while at first but it will speed up as you get better at the course and really help you to build a solid foundation of knowledge.
*This one is not for everyone but I find it useful* 3) When you are feeling more comfortable with the paper, and as the exam approaches, start doing the paper with less time then you are allowed in the exam to get used to doing questions at the right pace, then hopefully when it comes to the exam you'll have plenty of time to check over your paper for mistakes.