Short answer: Fully charge it once, then full discharge it once.
Long answer: It's necessary to condition a LiIon battery in order for it to perform optimally. They ship with half a charge, and if you proceed to use it like that and drain it, you will damage the battery and it will never hold as much charge as it should.
When a LiIon battery is first new, you should completely charge it once before you use it for any long period. (Turning the phone on for a few minutes won't matter; it's not that critical.) Also, within the first week or so you should let it fully discharge once by leaving the phone on until the phone turns itself off due to the battery being used up.
Once you've done that, you can largely ignore the battery; use it when you feel like, charge it when you can.
You may encounter people who will tell you that, when the battery is new, you have to "charge it for a full 24 hours". That's not really necessary.
The confusion there comes from the fact that the charging process involves two stages. In the first stage, the phone backflows current continuously into the battery. This is fairly fast and gets the battery up to perhaps 80% of its full charge. After that, there's a process called "topping off".
During the topping off segment, the phone moves current into the phone for a short duration, then pauses for a somewhat longer duration. The reason for this is that when the battery gets near full charge, it begins to heat up while current is backflowing. The pause is to permit the battery to cool again. (If this is not done, the battery can melt or explode. Kids, don't try this at home! That's why you should never try to charge a LiIon battery yourself; the process requires more than just a current source.)
This topping-off process takes an additional hour or three, depending on the size of the battery. Where the confusion comes in is when exactly the phone announces that the battery is "fully charged".
On some phones, "fully charged" means that the topping off process has completed. However, on other phones, "fully charged" means that the main charging process is finished, but topping off may still be going on even though that's been displayed.
Another source of confusion is an assumption people make that charging continues as long as the phone is connected to the charger. That's not correct; in fact, it would be dangerous to do that. On the contrary, once the battery is fully charged, the phone will cease charging it whether it remains connected to the external power source or not.