And another thing: hang on to your paperwork, returned quizzes, electronic copies of your papers, etc. etc., until the end of the semester at the least. None of that should hit the circular file before the course is over and your grades are in. (Or maybe even longer, if you're like me and have a sentimental attachment to that stuff you put so much work into.
You never know if the prof may have forgotten to enter your grade for an assignment and mistakenly think you didn't turn it in. In that case you can show the graded copy as proof. Even if you don't have that, you can print another copy and hope he'll take pity on you. Same thing for returned quizzes, tests, and so on.
At the very least, all that stuff is great reference when you're reviewing for a test or essay.
Also, don't be too quick to sell back your textbooks unless you're really sure you don't need it again. In a few cases you might need it again--I've used the same math book in two courses (using different sections) and I'd be kicking myself if I'd had to re-buy the thing.
And if you're taking a similar course in the same subject, you might want to use that textbook for reference/review even if it's not technically on your reading list.