Here's what you should do:

Perform shallow discharges. Instead of discharging to 0% all the time, lithium-ion batteries do best when you discharge them for a little bit, then charge them for a little bit. The table below, from Battery University, shows that discharges to 50% are better for your battery's long-term life than, say, small discharges to 90% or large discharges to 0% (since the 50% discharges provide the best number of cycles-to-usage ratio).

Don't leave it fully charged. Similarly, lithium-ion batteries don't need to be charged all the way to 100%. In fact, they'd prefer not to be—so the 40%-80% rule you heard is a good guideline. When possible, keep it in that range to prolong its life as long as you can. And, if you do charge it to 100%, don't leave it plugged in. This is something most of us do, but it's another thing that will degrade your battery's health. If you need to charge it overnight, use something like the Belkin Conserve Socket to stop it from charging after it's full.
Fully discharge it once a month. This may seem contradictory, but hear us out. While lithium-ion batteries shouldn't be discharged regularly, most modern batteries are what's known as "smart batteries", which means that they can tell you how long you have until your battery dies (e.g. "2 hours, 15 minutes remaining"). This feature can get miscalibrated after a lot of shallow discharges. So, manufacturers recommend fully discharging your battery once a month to make sure this stays accurate.
Keep it cool. Most people overlook this one. Excess heat is not only bad for your processor (and your lap), but your battery as well. Once again, see the table from Battery University below. A hot battery will degrade in health much quicker than a cool one. As such, we highly recommend using a laptop stand, like one of the many DIY options we've featured here at Lifehacker. When it comes to your phone, check out our previous Ask Lifehacker on keeping your phone temperature down.

Keep these things in mind and your battery will last longer. That said, you don't need to lie awake at night worrying about whether your battery's charging. Don't sacrifice practicality just to keep your battery alive—if you're in a situation where you don't have a charger, it's okay to discharge it to 0%, or charge it up to 100% if you want to do so for a long plane ride. Remember that your battery is going to die in a few years, no matter what you do—even if you just let it sit on a shelf. These guidelines are just that—guidelines to keep it healthy for as long as possible (and when it's dead, check out what do to when your battery doesn't last as long as it used to) http://lifehacker.com/5875162/how-often-sh...ng-its-lifespan