Make a commitment, before your schoolwork is done for the spring, to do some reviewing over the summer.
Create a study schedule for yourself before you leave school for the summer. Schedule in one or two hours per week, and stick to it. If you let it slide once, you're likely to let it slide again.
Keep your notebooks and books from your classes in a central place.
Get out your notebooks and browse through them. It's OK to be fairly passive: you're not studying for an exam, you're just trying to keep your brain engaged with school material.
Look over old homework and papers, thinking about how you'd improve them.
Thumb through textbooks, keeping an eye out for anything you've highlighted. If you've taken reading notes, review them.
Study just enough to maintain a baseline of brain activity so that you're ready when fall inevitably rolls around again. After all, summer is for play.
Pat yourself on the back when you're done and go have fun!