I think that an important thing in biology as well as in many other subjects is to understand the general concepts before you try and tackle the details. For example, you have to understand the basic ideas of diffusion, active transport, and concentration gradients before you can understand the unit on the kidney/nephron.
You'll find that such basic concepts are applicable in a lot of the different units and so sometimes you'll be able to use reasoning to come to an answer on some problems. That's not to say that biology has no straight-up memorization, but a good foundation of rudimentary concepts should make the job easier.