Here's a quick list of tips that will hopefully help improve both your studying habits and ability. I've done my best to keep these short in order to aid absorption and retention.

• A Distraction-Free Quiet Place: Study in a quiet, distraction-free place (such as an office or library). There are too many distractions at home (such as in a bedroom or open-plan kitchen/dining-room combo). Attending to distractions will cause a loss of focus/poor-quality studying and ensure you run out of time.
• Pace Yourself: Plan to Study for no more than two hours at a time, without breaks. (Breaks should be for a duration of no more than ten minutes each.) If one-hour study periods with five-minute breaks are better/more helpful, that's also fine. Cramming is neither efficient nor effective.
• Study Regularly: Many short sessions a week are much more effective than a few long ones the twenty-four or forty-eight hours before an exam. The best way to eat an elephant is with a knife and fork, taking many small bites.
• Proactive Preparation Counts: If at all possible, read the material to be covered the night before class. This gives you time to think about it and come up with questions to ask during class time. The aim of lectures (in a university/varsity setting) is not to teach the material to which you already have access, but to clarify and reinforce it.
• Discard Your Highlighters: Ditch your highlighter, belovėd though it may be. Highlighting is largely a waste of time. Your mind doesn't absorb information like a barcode reader, as fantastic as that would be.
• Mix it Up: Use more than one study technique. Reading a textbook and writing notes (a preferred method of many, myself included) is a passive technique. This is because it doesn't require critical thinking. While using passive techniques aren't necessarily bad things to do, they're not enough for information retention nor understanding. After reading something from a book or online, close it and try to explain what you've just read to someone else (preferably verbally). Also see if you can trace/follow any steps of deductive reasoning that lead to a conclusion. (Doing so is particularly useful for Maths and Science disciplines/subjects.) Active techniques such as this will strengthen the logic pathways in your brain.
• Do the Exercises: Work through the problem exercises that are presented in the text, usually at the end of a chapter. They're there to help you determine if you understand the presented material and can apply it. They also help to reinforce it. Don't peek at the answers until you've had a good go at answering the questions, either. There's no point in leaving it to writing the exam to find out if that (the former) is the case; by then, it's too late.
• Decide what's Important: Since time is often limited and you cannot always learn all the presented material (even if you would like to), you have to focus on the important parts. Usually, what a lecturer covers and assigns as homework/assessments is what's important.
• Don't Panic!: When it comes to answering questions (either as homework or during a test/exam), first look through the questions and decide how you're going to handle them before getting stuck in. (You don't have to tackle things sequentially.) If you get stuck on a problem, you can always move onto something else and come back to it later, rather than sit spinning your wheels. It's probably not as difficult as you think/feel it is, but having a block against something doesn't make that apparent. Shifting focus and coming back can sometimes help one to see the simple solution later on.
• Multiple Choice Gotchas: After reading a question with multiple answers from which to choose, try to think of the answer before reading the available options. Some almost correct (slightly wrong) options can be confusing and lead you away from the correct one.
• Ask for Help: Don't be shy about asking for help if you need it. Admitting that you have a problem or an issue or don't understand takes strength. Sitting in the dark and gnashing your teeth, getting nowhere, is of no use to anyone. Everyone needs help sometimes and nobody knows everything, so take advantage of what you can get.

As stated prior, the above points are condensed in order to be easily absorbed and retained. For those who want an expansion (and have the time for it), I recommend Marty Lobdel's talk/lecture (below):

I recommend watching this at 2x speed, so that your mind doesn't wander onto other things. Otherwise, watch the five-minute summary: