I keep making silly mistakes how do I stop that?

That was always my biggest problem with my GCSE maths, I would get excited when I thought I knew what I was doing then slip up with silly mistakes. One of the best ways I found, after checking over at the end of course, is right down some of the important rules you know down on the front page before you start, the ones not on the front of the exam paper. I always used to write down BODMASS, the Square root of a number can be plus or minus. I always used to write down a few of the circle theorem rules that I would forget later on too.