For chemistry, I read whatever is in bold face first and then I go back and read all the text, highlighting the most important information. To memorize chemical reactions, I use index cards. I can take them wherever I go and look at them whenever I have free time. I see a tutor twice a week.
When reviewing a chapter, make sure you understand the diagrams, pictures, and charts. Diagrams are the most important thing.
Do some looking around in the bookstore for good study guides and review books.
When reading science books, make sure you read the examples because that's how you understand. If you don't read the examples you'll be lost during class.