How to best prepare for the AP Biology exam?

Study what you're not familiar with.

flash cards and study groups work wonders.

I say u study both parts because you never know what your teacher will put on the exam and most likely the lesser parts will be worth more points than the bigger parts of the exam.

Your best bet is to study one of the AP review books.  This with your text book is the best way to prepare for the exams.  The review books have sample exams with solutions and study techniques that help significantly with the exam.  You really need to work your way all the way through the review book and be sure to figure out the stuff you do not know when doing the sample exams.  Many students like to study the stuff they know because it makes them feel good, but it is the stuff they have problems with that will lower their score.

I have put together a collection of the Best AP Review Books for each exam at http://www.APReviewBooks.com.  The Biology books are found at http://www.apreviewbooks.com/index.php?id=18.  There are also links to the released questions from past exams.  Going through these will help you see what has been emphasized in the past.

Good luck on your exam in May.