It is important that you create a study plan. By this, I mean to get a parchment or paper, whichever you prefer, and write down everything you want to study. Be sure to put them in order of importance. For instance, if I have to study Herbology, Charms, Potions, and Astronomy, and I have a Charms exam in two hours, an Astronomy exam in 5, a Herbology exam the next day, and a Potions exam after the Herbology one, I would list it like this;

1) Charms
2) Astronomy
3) Herbology
4) Potions

This will ensure that you are guaranteed to study the soonest ones first, then the later ones afterwards. As you study the required subjects, mark them out or check them off so you know to move on to the next one.

It is always important to have a plan before you begin doing something, even if it's just studying. Having a plan will make you much more productive.