From Larry: The best batteries that I've found, that last longer than any others, are a brand called Maha Powerex batteries. There are different mAh levels you can choose from. I currently use the 2700 mAh batteries. You can get these online at Thomas Distributing. They also have a charger that will charge 12 batteries at one time made by a company call Ansmann. It is a smart charger that senses what the batteries need before it starts its charging pattern. I find that I can charge up a whole set of batteries a few days before an event and they hold out. If you are still friendly with the SmartShooter crowd, they offer a discount at Thomas Distributing.
And from Ivan: Very useful post, David. However, with good batteries you also need a good charger... like the La Crosse Technology BC-900 AlphaPower Battery Charger at Amazon. I don't have it yet, but David "Strobist" have been raving about it.