I've been using Imedion batteries in my 580EX II for almost a year now. They hold their charge very well since they're low-self-discharge, so they won't die quickly if you keep them in your bag. They also have a pretty good life at 2400mAh. Couple that with a PowerEx charger to make sure they aren't cooked by those fast chargers and you should have a system that lasts quite some time. You'll probably still want to keep around a spare set, but I've gone over 800 shots on just one set (almost always less than full dumps, since I don't want to roast the 580EX II), and then I just switch out to the next, get home and put those 4 on the charger. If you're doing a long shoot, take 2 extra sets.
The other thing you can do is get a speedfire to run it off AC, or something like a quantum turbo to give yourself a much bigger battery. Those tend to be pretty good solutions, although they are more pricey. The upside to them is a faster recharge time on the flash. You still need to use internal batteries to power the rest of the flash's functionality.
