Li-Ion batteries are a strange beast compared to the nickel based rechargeable that we've probably been ingrained into thinking how we should handle rechargeable batteries.
For starters, there are the two ways to prolong Li-Ions
- If you're not using them for long periods of times, keep them as cold as possible. The lose less energy this way.
- Allowed charge cycles depend on how low you discharge it first. At best case, 100% discharge allows you 500 cycles. 25% discharge allows you 2500. If you do some math, you get more energy out of the battery if you recharge as often as possible
So contrary to what kleinkenstein said, you should actually keep Li-Ions charged as much as possible. Leaving the plug in won't harm it as the charging circuitry will shut off when it finds the battery is full.
Keep in mind that Li-Ions lose max capacity the moment their out of the factory anyway.