The simple answer is to use the heater only when you need the water warm. From a thermal standpoint, heat transfer is proportional to temperature difference and in this case, water and air. Stated another way, you lose less heat if you allow the water to cool down and get closer to air temperature than keeping the water well above air temp. This ignores the convenience factor of being able to jump into comfortable water at any time. If you don't use the pool for several days then by all means shut down the heater and save $$$.
Al
Edit: Looks like Mark and I were typing at the same time. Same ideas.