Revise a bit of GCSE, particularly if there's a subject you haven't taken for A-Level (eg Physics for me.) Look over the BMAT book to familiarise yourself with the style of questions, but more importantly, don't get too stressed about it! It's an aptitude test. The most that preparation will accomplish will be to make you sufficiently relaxed and confident to work through challenging questions with a tough time limit.
And DON'T shell out vast money on a course that claims to boost your scores. For one thing, I think it undermines the system, and for another, it will help very VERY little. I spent no money other than the entry fee for my BMAT, and I got 9.0, 7.7, 15.0.