In my experience, it is *pretty* effective to discharge to a point of stability (this might get higher and higher as the battery gets stronger).
In other words, turn the lights on for two or three minutes until the displayed voltage levels off. Terminate the discharge and monitor voltage until it rises and "levels off" again. Note the voltage. If under 12.65v, recharge (actually ->charge with sufficient amperage and voltage up to, but no more than 15.0vDC.)
Monitor how long it takes for the battery to drop back to 12.65v (should take longer and longer with each *discharge-charge* cycle.
Be aware of the temperature adjusted voltage values.
[oh, and be certain your battery terminals and grounds are clean and sealed. Caig DeOxit rocks!]