Study-guides
I am not entirely confident that all (high school levelled) educations have published study-guides, nevertheless I can tell you IB, for example, does. They can be found at the IBO´s website store (www.ibo.org), or bought used on ei. Ebay. Instead of reading the entire school book again right before exams, you can get through a study-guide in half the time, and refresh all the important information, formulae, points etc. Saves you time (but not money). The only bad side of a study-guide, is that it's not particularly useful if you haven't already read the regular book/attended classes, or if you're having a hard time understanding the subject (if so, remember to check out khans academy!).
Khan academy (www.khanacademy.org)
This website saved my life as far as biology goes. If you have difficulties understanding a subject in general, no matter how much time you spend studying, or if you need a quick recap of a subject. Khan academy is basically a website full of free online lectures, and has excellent standards. While they do not always go completely in depth, they explain the basics of all you need to know, in a way which is incredibly easy to understand. Personally I only used Khan academy for biology, but the also have videos on many different subjects incl. mathematics and economy.
Past exam papers
Reading the books themselves aren't always enough if you want a good grade. You also need to be used to/understand the exam´s form. Therefore, ask your teacher (or google) for past exams papers in the subject, and do these papers, preferably with access to the mark scheme for the papers afterwards, so you can check your answer, and see what gets you the maximum mark. You might have to use a few minutes to find the websites with the right exams papers, but it is definately worth your while. My personal preference is www.xtremepapers.me , it has papers from all different educations and on many different subjects, often with the mark scheme as well.
Bedtimes
Go to bed in good time. Don't pull an all-nighter studying the night before your exam. It can only work against you, because buy the time your exam starts your brain will be working slower than usual! You also have a harder time concentration on the subject. Set a minimum of 6 hours of sleep. If the subject of your exams is one you have difficulties with, 6 hours of sleep is acceptable, as it gives you two hours more of studying. If it's an easier of the subject, sacrifice the two hours of studying on sleep, so your brain can process at the knowledge within the time frame of the exam.
Stay sober!
For some of us, exam preparation temps to drink (the nerves away). But for goodness sake don't! A buzz will only make it more difficult to remember what you studied, and rob you of your energy the next day. Whether we like it or not, alcohol kills braincells, and therefore alcohol does not go well together with your studying. Have a piece of chocolate or some soda instead to help calm and paper yourself a bit.
(Try) to make a plan
When your study-break start, attempt to make a plan of your revision period. What do you study which day. How many days are you going to study during the week? How many hours of the day? What time of the day? But while doing so remember to be realistic: Try to take account for potential procrastination in advance, prepare for unexpected events (visit from family, getting sick, etc.), and give yourself a break once in a while if the study-break is long term (2-3 weeks in a row or more), to see friends and family and put your brain on hold for a moment.
Check your syllabus
What do you actually need to know, and what is just filling in your books? Especially American schoolbooks tends to be bigger than necessary, because the authors gets paid for the quantity instead of quality. But in European (and many other) countries part of what is in the school books is non-important information. Also some books has both the high level and standard (and lower) levels content in them. So if you're, for example, standard level, there might be content it the book you need not know. The easiest way to keep yourself from wasting time reading things you do not need to know, is to read and follow your syllabus. Usually the best way to get the syllabus list, is to simply ask your teacher, as they are always given your syllabus.. even if they aren't smart enough to share it with their students.
Know the differences between the exams
In IB you only have written exams during your exam period (and 2-3 during the 2 years in IB). In all subjects you have 2-3 exams: Paper 1, paper 2 and paper 3. What you need to know for the different papers (which are usually on different days) tends to be quite different areas of the syllabus. Therefore there is no point in preparing, for example, the content of a paper 3 exam for your paper 1 exam. Therefore be sure to know what it is they want from you in the different exams you have to go through, regardless of their structure (written paper 1-2-3 exams, or a combination of oral an written exams, etc.).
Eat something
What ever you do, don't let your blood-sugar get low in exams, it will make you tired and unfocused. Eat some kind of breakfast in the morning, even if you usually don't. If you're too nervous or too tired to eat proper breakfast, eat some fruit. Generally try eat a healthy breakfast, rather than a sugar filled one, because with a sugar filled breakfast your blood-sugar might rapidly increase, but it will also rapidly decrease, and you don't want to end up with super low blood-sugar in the middle of the exam.
Move on
During your exam period take it one day of the time. Prepare for you exam, take it, and then - regardless of how it when: good/bad/okay - MOVE ON and focus on the next exam. Spend maybe an hour after the exam processing it, discussing it with others, cope, etc. and then move your focus on your next exam and don't linger on it. What´s done is done.
Best of luck to everybody who´s going through either their exams or their study-break at the moment! :-)
Speaking of the devil, I have to get back to studying for my biology exam tomorrow. 2 days 'till graduation!!