When I applied I had been out of school for four years and had a lot of work to do to brush up on my sciences. I started at first by going over my old text books, but I soon switched to studying my sciences from
Schaum's Outlines of Biology,
Chemistry, and
Organic Chemistry. At the same time I took the
Kaplan DAT Class. It was expensive but definitly found it helpful having been away from the sciences for a while. They layout all the specific areas you need to know for the DAT and you have an instructor that you can ask questions to. If the class is too expensive, you may want to just try the
Kaplan DAT blue book as it has a lot of the material from the class. There are other resources out there as well, but that is what I found helpful. I also used the Top Score Pro CD to give me some really accurate practice tests during the last few weeks before taking the real test. Good luck.