How can I improve battery health on my MacBook Pro?

I’ve been keeping tabs on my computer using the iStat widget and I noticed that within the course of about a week, my battery health percentage, however the hell that’s calculated, has dropped from about 96% to 61%. This number’s backed up by the rapidly falling lifetime on each charge. I’ve only owned this computer since August ‘07, so it’s not an old fart or anything. Is there anything I can do to repair this issue, how could it have happened so fast, and how can I prevent it in the future if I end up needing to buy a new battery?