My freshman year, I made the mistake of buying a 1 and 1/2 inch for each of my seven of my classes. Not only was it a pain to carry around everyday, I ended up not needing a lot of the extra space. This year I was wise enough to simply get two separate 3-subject notebooks. While it's fun to buy a lot of different binders and notebooks in fun patters over the summer, keep in mind that you'll be regretting it later.
If your school is fortunate enough to have a class set of textbooks, then you're one of the lucky ones. Unfortunately, all of us aren't. When managing your textbooks, think: What do I need in class everyday? Which books are available online? If you aren't required to bring the book to class, why lug it out there everyday? As long as it is not interfering with your grade or classwork, it is perfectly acceptable to leave you books in your locker or at home.
You want to purchase a sturdy backpack with wide straps, because trust me, any other will leave your back aching. Girls (because I know some of you are shaking your head through this), if you absolutely must carry a tote bag instead of a proper backpack, at least get a large one so it can hold all of your supplies, and one with sturdy, broad straps so that it won't be cutting into your shoulder.
Small things add up! Only bring your calculator on days that you need it. Bring a small water bottle, and just fill it up during the day. Take out unnecessary items (girls, is it really essential for you to bring a full size mirror and brush in your bag?), and leave them at home. Remember, you're just going to school, not on vacation. Guys can leave their two-ton wallets at home (a few dollars and your i.d. should be plenty for a day), and girls can leave their enormous makeup bag at home (At the most, a tube of concealer and lip gloss should be good).