I second biosci3c; while prep books might be a good investment for other subject tests, they're not worth it for the Biochem GRE.
As far as I know, there are only 2 official practice exams available from ETS: the 2005 version that they send you after you register (you can download this directly from the ETS website) and the 1994 version that's floating around on the internet somewhere. Use them wisely! I spent a few weeks reviewing the topics on this site using a combination of textbooks and notes from my undergrad classes, and then took one practice exam. I went back through my exam and made a list of topics I needed to study more based on my wrong answers. I repeated this for the second practice exam. If I had more time, I would've covered anything that the practice tests had missed. In the end, though, this strategy got me a 90th+ percentile score.