Get at least one full cycle of sleep. Cramming leaves you sleep-deprived, disoriented and delusional. Staying up late and getting up early is fine, but pencil in at least three hours for some quality REM.
Read chapter summaries and class notes. Don't try to tackle the whole textbook if the test is the next day. This will never work! Not only will you forget most of what you read, but you also won't remember enough specific details to answer questions. Chapter summaries highlight the most important aspects, and class notes provide great clues as to what you might be tested on.
Eat energy food. Fruits, peanuts or whole grain crackers are good snacks. Chewing gum can be helpful, too. Although you'll be tempted to guzzle caffeine or go for a sugar high, both of these options will make you crash--and more than likely, it will be right around the time the exam starts.
Take a break. Your mind needs time to rest, so take five or ten minutes every couple hours to rejuvenate. A brisk walk helps, as does a specific task, like making a cup of tea or starting a load of laundry. If you only have a couple of hours before the exam, take at least one break.
Be upbeat. The thought of impending doom will not help you succeed on an exam. Chill out and tell yourself you're going to be fine (whether or not it's the truth). A calmer mentality will help your brain concentrate on drawing from memory for answers.
Think critically. By far, the best way to cram for an exam is to think critically about the concepts and theories covered in the class. Since you don't have time to study all of the material covered, thinking on a broad scale about the purpose of the class may help you answer questions without really knowing the answers.