Barron's AP Bio flashcards are excellent! Super helpful and inexpensive! I have both Cliff's and Barron's. Definitely make sure you know the various biological pathways: glycolysis, krebs, etc. What goes in/out of each cycle (glucose--pyruvate--acetyl CoA) What is being reduced/oxidized (NADH-NAD+, FADH--FAD+). Also know oogenesis/spermatogenesis. Basic Genetics, photosynthesis (light and dark reactions), etc. Know the basics and details for pathways, etc, and you should be fine! Cliff's is just fine!