As Kevin said, it's a good idea to let it run down then fully charge it. Apple recommend doing this only once a month though. The iPad batteries are modern in design and do not suffer from old problems like battery memory. You can charge them as little and often as you like, but give them a full cycle as described about once a month.
Here is Apples page on iPad battery info, with that advice at the bottom -
Apple (United Kingdom) - Batteries - iPad
Since the iPad3 battery is so massive, it does take a long time for a full charge. I find it best to do this overnight. There is no danger to the device or the battery in leaving it charging, even once it is fully charged.
Hope that helps.