My advice for any math class is the importance of understanding concepts or ideas, not memorizing steps to solving a problem or memorizing formulas. Sometimes memorizing a formula may be helpful because you can solve a problem faster, but make sure you understand why a formula works and why you should use it in a certain situation. If you don't understand a certain topic, make sure to ask questions to understand it. Do not just let it pass by, thinking you won't see it again after a test--they will come back.
Some topics I remember learning from pre-calc are limits, trig, transformation of graphs, logarithmic and exponential functions, inverses, domain and range of functions, parametric and polar equations, etc.