I guess you just have to be confident with what you do.
Thorough checking of your answers may help, but if you check too hard for too long, you may end up changing a correct answer to an incorrect answer (this happened to me for one of the MCQ on the exam).
Being competent and making sure you understand what's being asked is very important.
Answering the question properly may be difficult for some due to the inability to comprehend the question (for example I remember in the '10 exam 2 a question in the core asks for the error (i.e. difference) as an answer and consequently many people failed to recognise this.
Make good use of your calculator to check answers which you've done by hand.
But most of all, be calm. As we know the further course itself may not be tricky, but reducing silly mistakes may be extremely hard.
It's a combination of competency and luck (to a lesser extent!) in this exam, and I wish you all the best
Good luck.
As a side note, usually your first answer is correct, if you're unsure whether you should change it or not (as in on the fence for two answers), I suggest the latter