Biology: Very hard. usually have to take notes because I'll read the stuff and forget everything I read (or at least, forget the stuff my professor will test me on).
Physics: Read it, get the concepts, and then focus on problems. I've found that most of my understanding comes from doing problems rather than actually reading (for some of my upper level physics. For the general physics classes, usually reading was enough and a few problems may illuminate some weakly understood topics).
Chemistry: Read through, then probably read through it a second time sometime later. Problems weren't as essential for me (except for organic...had to do problems for that).
Math: Problems problems problems.