Here is a good review of the science of charging these batteries:
Maximum Battery Life - BlackBerryFAQ
My habits (which seem to work well for me and others):
New BB: Charge full to 100%. Drain to 15-20%, recharge full to 100% about 2x -3x in a row. After that, let it drain each day, with topping it off anytime you get a chance, 10-15 minutes or longer anytime you are near a charger.
At night I let it charge full again.
About once a month, let it drain to 15-20% again, a couple times in a row, rully recharging each time to 100%.