Definitely know everything on your course inside and out, and do every single past paper
Also, you need to really understand everything and preempt possible questions, since especially at A2 you get a lot of 'suggest' questions, and papers can be really synoptic, so stay on top of stuff you've learnt in AS as well! (This is especially for science subjects) and get used to answering questions on completely new things, which usually link in some way to something you've learnt.