I'll toss in my bit here.
It's all a question of "low impact gaming." With each session take a long hard look at what you use and what you don't use. If you don't use if for two or more sessions, leave it at home next time. Sure, every gamer comes across the problem of never knowing what you might need, but traveling light requires some hard decisions. And is it really worth carrying around a 5 pound book when all you get out of it is a single page of information? You have to ask the question and answer it honestly. It's only your own back you're breaking, so who's getting hurt if you're not honest?
1. Take the core (assuming RMSS which is why I like RMSS's organization and hate FRP's organization): RMSR, AL, SL. That is all you'll need.
2. Keep a journal of the stats of all the monsters you'll be using that session. Over time you'll collect big enough journal of all the monsters you use (without the ones you know you'll never use). Now you can leave your Creatures and Monsters at home.
3. Do you really roll up random treasure at the table? If so this one may not help. If you don't just pre-roll all your treasure. Treasure Companion off your back.
4. How often do you need Races and Cultures
during play? Probably not alot. Leave it at home. You know what a dwarf is.
Go through this process with each book and as you find more information you use in the planning phase and not at the table you'll find books you can leave home. If it ends up being one page you use over and over just print that page or hand copy the need info. If you're caught without the info you need (trust me, it'll happen only rarely after this process is done) just wing it.
Using FRP? Well, the organization for that was made for more of a one-book-game, but in the long run it makes transportation more difficult.
As always, just suggestions.