This tip has saved me on many tests- the first step to this tip is to keep a HUGE binder at home (aim for at least a 2") and divide it by subject. The second step is to, one a unit is over, put everything you used for that unit in the binder, preferably in chronological and another type of order (for example, syllabus, homework, handouts). This ultimately keeps your binder uncluttered and helps with studying by keeping everything in the right place. For the midterms, it will also be easy to just study from the big binder. And, as an added bonus, a little weight comes off of your backpack!
This is a nice idea. It's good to keep things until you know for sure you won't need them. You might use them for another class, and wish you had kept things. Keeping things in a binder can be difficult though. You might want to try a small file cabinet.
I do something very similar to this. I keep file folders full of the year's work. Its like a persona archive. At the end of the year I throw out anything I don't need (like old vocab quizzes). I take special care to hold onto graded papers, math and physics exams, and teacher handout notes that cannot be found elsewhere.
This is a great idea because I myself have done something like this and everything was much more organized. I even keep folders for my other classes once I get tests and sheets back that I could need so when I need it I can pull it out instead of rummaging through a pile of papers.