1. get your books rebound into smaller sections. i divided my big casebooks (civpro, torts, contracts) into 3 sections. they will do this for you at places like office depot for about 4 bucks a section. totally worth it. they bind them nicely and reprint the cover of the book, then i write what pages are in that section. it has helped me carry soooo much less, since i only bring my mini-books that we're actually reading.
2. get a book stand for reading. you should not be reading by having the book in front of you on a table... if anything pile it on top of your other books so your book is higher off the table. i have one book stand that stays at my apartment, and one that travels with me.
3. get a backrest. non ergonomic chairs suck. my law library lets you rent out backrests/seatcushions, which i do all the time and take it to class.
4. get a chiropractor if you can. they're often covered by your insurance (mine is, thank god.) i wouldn't survive without it.
5. as someone who has always carried stress in my neck/back/shoulders, as well as having a history of back problems (thanks mom & dad), stretching has reeeeeally helped me over the years. if you google back/neck stretches, you'll get tons of stuff. its easy and you can do them in the library (ie. get up, walk around, stretch out, etc).
-------but yea, i totally hear you on the back/neck/shoulder pain. law school - it hurts!