I assume that you are talking about the Maths GCSE. Silly mistakes are incredibly annoying and nobody is able to escape them. My advice for you is, when doing your paper, ensure that you look back at each question, and within each question, redo each calculation and look at your logic process for it. Slap yourself if you need to - just ensure that you're wide awake.
When I did my Maths GCSE, during the exam I made a horrific mistake on every single question, but because I went back and looked over it (pretty much did the exam paper twice over, as I got more comfortable in the examination room) I got a grade I am happy with.