Reading you remember about 20% of what you read.
Read and process, by process I mean one or both of the following:
Speak. Speak it out loud, try to explain it to yourself as if you were a teacher or a lecturer. Read and then try to explain it to yourself without looking at the textbook. Get back to the textbook afterwards and see if you missed anything and/or explained something slightly wrong. Additionally, teaching others is an excellent way of getting things to stick in your mind. The teacher always learns more than the student.
Write. Write down notes once looking through the textbook, and then write notes on everything you have just written/read without looking.
This is what I did at A-Level. A-Level is about 80% memory, 20% knowledge in my experience with the Sciences. So these techniques are pretty useful and extremely useful at University for certain topics that require a slight emphasis on memory.
Science specific:
Go through derivations of formulae, get to know your formulae, where they come from, what's significant about them and their assumptions, (More of a university thing, but good to get used to at A-Level).