If you dont continue to use something (or read something, review something, etc) then you are going to forget it. Its that simple. The human memory really isn't that good. I mean you arent going to TOTALLY forget it, it should at least sound familiar if you see it again, but it just takes a simple review and the material should start coming back to you quickly. For example if you look at comprehensive finals. Yes, they are scary. However, if you have been keeping up all semseter and been studying hard then the review should be pretty easy and youll find that the little stuff you forgot will come back very quickly. Really the only strategy to remember something is to keep reviewing it. If you leave it alone for X amount of time, eventually you will forget it.