+1 to the sport drink powders: lets you mix to the concentration that you like, and is much cheaper than the pre-mixed stuff!
Water is heavy, so I usually don't fill my camelback completely. I'll run about 1.5 L in the bladder, and bring a 2L pop bottle full of water onto the field in my backpack, plus a bottle of gatoraid. (Won't put it in my hydration system.) I've usually got another 2L H2O back at the car. I'm a type 1 diabetic, so I always carry some form of carbs: granola or fruit bars, usually: in my vest so I'll have them if I need them. I also carry a sandwich (often a large Safeway lumberjack style) in my backpack, which gets left at the re-spawn point.
A suggestion for those who are really having electrolyte issues: carry a few small salt packages with you (the postage stamp size things you get from fast food places). This provides an easily accessable, small dose of salt should you need it.