The best thing to do is when using the battery, let it run down to somewhere around 3% left and then charge it backup. The one thing a lot of people forget to do with any notebook (Apple, Dell, Gateway, Sony, etc) is to condition the battery. To do that, run it down to nearly dead (3% for example) and then charge it back up. If you do that then it will get used to using the entire battery.
A lot of people also only use the battery part of the way all the time and then charge it back up. So say for example you only let the battery get down to 30% left and then plug it in all the time. If you always do that then the battery will get used to only working under 30% is left and then the battery life will plummet to 0% and the notebook will go to sleep until you plug it back in.