I suppose as a history major I have a little experience with this. My suggestion is to rely on repetitious reading. If you have notes on the subject, read them from beginning to end at least five times. Then go through your textbook and skim it, starting with the large events (section headings and what not), and then eventually taking the time to pinpoint certain areas of major importance (D-Day, or the Nuremberg trials, for instance).
Honestly with history it is all about memorization, and the most effective tool for memorization is repetition. Read, read, read, and read some more. Break events into lists of chronological order and memorize dates, names, places, etc. Then read, read, read again.
It also helps to try to associate the names of famous men and women with photos of them, if you can find any.
I guess that's about all I can offer. A lot depends on your own abilities and what works for you. Some people benefit from study groups, while some people need to study alone. Good luck!