Maintain battery life
All rechargeable batteries including a e-cigarette batteries will lose life (ability to recharge multiple times, amount of charge per cycle) if the charge level gets too low. Do not allow the battery to get discharged all the way.
When if an eCig battery gets drained out do not drain it till that last puff. Even if you do make sure you charge it up right away. If you do not this is a sure way to kill your battery performance. A good solution to avoid drain outs could be to get the longest battery you can comfortably carry.