Often when students study for an exam, they read through their notes, highlight passages, skim their textbook. This kind of "studying" doesn't help in retaining information for a test. Pausing work at different intervals and asking yourself to explain what the information is can show you what you've really retained. For example, read 1 page of a US History book, cover it, and see how many things you can remember from that section. These techniques take a little effort, but provides a great result! This NY Times article explain how memory works.