The best thing to do before having a final exam? and yes i'm giving this question 96 points so it must be a long answer. have fun with your points : )

Well, there are many different opinions on this matter. Some people believe that studying until the start of the exam is the best way to go. This increases your chances of having topics in your exam which you read about just minutes before starting the test. This won't really work though if you have hundreds of pages to memorize.
Other opinion is to have a good sleep before taking a final. It is a commonly known fact that sleep helps storing knowledge in your long-term memory. Since sleep is always beneficial, this is a sound strategy. But if you don't want to actually keep the knowledge, just pass the exam, then you can choose the first method, and study study study until the exam starts.

Other useful things:
Eating: Chocolate and other stuff filled with a lot of sugar can help boost your brain functions. This is a short term solution only. If the exam is long and you are not allowed to eat anything during it, then avoid eating a lot of sugar before it. Do not eat fat-rich stuff either, it will "weigh you down", you'll become sleepy and it makes harder to concentrate.

Drinking: Although sugar is useful, drinks with a lot of sugar in them should be avoided. Drink water or fruit juice, being thirsty during an exam is a torture if you are not allowed to drink and can ruin your performance. 

Exercise: Might be useful, but don't overdo it. A short workout is good to help the blood flowing, but getting exhausted is a good way to strip you of your chance to ace the exam.

Drugs, herbs, etc:
Of course there are many enhancement drugs and herbs which can help you, they are usually not worth the price. Some people take sedatives to help stress, but this should be avoided at all costs unless you're the kind of person who is completely blocked by stress. Sedatives make you unfocused and will be unable to solve problems which require complex and logical thinking.

Socialization: It is a great way to achieve a good grade if you find a few friends and study together in a calm, friendly and enternaining environment. Talk about the topics you need to learn, ask questions from each other. When you answer a question, find arguments to support your opinion or simply talk about a topic, it helps you to memorize that knowledge and will be easy to recall it during the exam. This method has helped me many times and I find this to be the most important thing to do before an exam.