You have to let them fully drain before letting them get fully charged. Don't leave them on the charger after they are charged. But really, battery lives don't last that long and they do need to be replaced every 1-2 years. I have to get a new laptop one sometime. At least I know I can get them cheaper on eBay!
That being said, I'm not sure if you can get them back to keeping a good charge once you've already screwed them up. LOL