Do you mind looking like a geek? If not, get yourself a wheeled backpack. CalPak makes some really good ones. I started using one after I had shoulder surgery midway through my 1L Fall semester and I haven't regretted it. It's held up pretty well to the daily abuse, being hauled from classroom to library, from the car to the conference rooms. Casebooks, supplements, snacks, even a legal dictionary... it all goes in there.
Yeah, I know what it looks like, but the convenience is well worth it.