unless it's an old laptop with a nickel based battery, you're better off letting it plugged in most of the time;http://www.dansdata.com/danletters017.htmhttp://www.marco.org/2009/09/24/laptop-battery-myths
granted the battery will slowly degrade, but it's a better choice than letting it repeatedly drain power and re-charge it repeatedly. personal experience; my one year old laptop used to be able to handle 6 hours with the battery. nowadays, it'll go for 5.5-5.75 hours.
if you're crazy extra serious about prolonging the battery's lifespan, then there's keeping it in an airtight bag and putting it in the fridge