Practice, practice and some more practice.
But DO NOT plan to spend long periods of time working that will leave you feeling tired and unmotivated, that is just unproductive. Schedule breaks: go for a run, or do some other physical activity. Eat well, sleep well. Minimize your stress. As to for what you should be studying: solve problems. Your brain may not physically be a muscle, but it acts just like one: the more you train it in a particular activity, the better it gets at it. So, if you are preparing for a big exam, get sample exams and solve them. Sure, you should look stuff up so that you understand the concepts (the more you understand the general concepts of the area you are interested in, the easier it is to build on that and remember new things), but in order to cement what you learn, do some examples. If you get a definition, try to figure out if that definition is reasonable (again, by example is usually easiest)