One of my problems with batteries is that they die (and that they're so dramatic. Other electronics break, but not batteries. They die. They were living, electricity feeding things that are now dead. It's sad.). There are solar charging circuits that you can buy from adafruit (note, this product is being redesigned, so the link may not work in 15-20 days from this posting, according to adafruit), which has great tutorials on how to use their products along with how it's built, but I also found another circuit at electronics-lab.com that has a solar charging circuit. I'd like to make solar powered robots, but for now I'm just buying lots of 9V and AA batteries. Going green will have to wait until time allows.
Microchip also has a solar power charging paper, which goes over the use of their chip designed for use charging batteries using solar power. Here is the article.