Every time you use it, using an automatic electronic charger.<br />The less time the battery spends not fully charge, the longer it will last. Try not to discharge it past a 50% charge (not 6 volts!) and it will last a very long time.
If you need to go out and buy a battery charger, spend a little time researching and a few more bucks and get a decent charger either dedicated to deep-cycle recharging, or with a deep-cycle setting.
