I was also a little rusty in bio before I took the DAT. To prepare, I read the bio section in the Kaplan book, read relevant chapters in an old intro to bio textbook I had, and made flashcards of the concepts I learned.
To prepare for the math section, I read the math portion of the Kaplan book, but I heard that others on SDN found it helpful to study the math sections in old SAT and GRE books.
Although these study methods worked for me they may not work for you because everyone has a different way of learning. Some people learn better if they practice and others learn better if they read more. Find which method works best for you.
If you are a little rusty in bio and math don't worry because you have plenty of time. I started studying just 2 months before the exam and I did just fine.