There are lots of resources for you to practice for the AP Biology test. (Look at these sites for other practice tests for other AP tests too.)
The official practice tests will be the most useful, as they will be most like the real test. So if you have limited time, use those first. The other practice tests will be helpful too; however, sometimes they can be way harder or easier.
OFFICIAL
Practice free response questions
Practice questions(from the curriculum guide, scroll down to find them)
1999 Exam
2002 Released Exam & Answers
2008 Practice Exam & Answers
NON OFFICIAL
Both of these websites list a bunch of links to AP Practice tests, questions, study notes, and free response questions.
AP Practice exams
Test Frenzy
