Definitely buy Cracking the GRE Math Subject Exam from Princeton Review. Go through it thoroughly, it has a good review of most of the things you need to know written in ways that are practical for you to learn before the exam on your own if necessary.
I would strongly suggest you dust off your old Calculus textbook. Skip any sections with word problems/applications/etc, but otherwise do all of the computational problems (i.e. limits, integrals, derivatives, some multivariable problems, especially Green's theorem type) as quickly as possible. The subject exam is extremely difficult because you have so little time to do problems like these. By doing a very large number of problems quickly, you'll find yourself developing the killer instinct for solving these problems without wasting valuable time. If you have a good diff eq book, I would suggest you do a little practice out of it too. Lots of work will also improve your accuracy. It's important to remember you can get a relatively high score while answering only ~50-55 questions provided you get them all right.
I would spend less time on the exams that are available online except the one available from ETS online currently (you'll also receive a copy by mail). You'll find that since rescaling happened, the exam in the back of the Princeton Review book and the current practice test are significantly more difficult than the older exams which are available online, but they can be a good resource for some types of problems you'll see.