Studying is hard work. And there is no magic way that works for every single person out there. The way that you study can depend on the subject matter as well. For math, I believe that the best way to study is through repetition of the types of problems that you will encounter. The repetition will help build and instill confidence in your ability to solve problems, and you eventually develop habitual skills about what actions need to be taken next depending on the types of problems being asked.
If you are studying for a literature test, you need to spend time going over the "guts" of stories. That means for any possible story/poem that might be on the test, you need to know what the overall plot is, who the main characters are, what the conflict is, where the setting takes place, and what the themes are. In addition to knowing the themes, be sure to take time reviewing and finding sections of the text that support each theme. Practice writing out compare and contrast lists of characters. That will help you study, and it will point out gaps in your knowledge.
For other subjects like history and science there is a lot of facts and vocabulary to know, so memorization of names, dates, and parts is critical. Some people are good with that kind of thing, but many of my students find large amounts of success with flash cards. Flashcards are an easy self quizzing tool, but they also have the benefit of being able to be handed to somebody else. That person can quiz you.
There are other subtle ways to help yourself study as well. You want to study in an atmosphere similar to the atmosphere that you will be taking the test in. That means you should study at a table or desk in a quiet room. Without listening to music. There are lots of studies out there that link memory recall ability to environmental ques. Your testing environment and your studying environment should compliment each other in their similarity in order to trigger those ques. One last weird one. Study with a mint and take the test with a mint. There are a number of studies that have linked mint to increasing cognitive function. I've linked one of them.