Use a solar pool cover
Install a pool heat pump
Use a gas pool heater
Use an electric resistance heater
Add solar pool panels
Run the pool pump during warmer daylight hours
Reduce heat loss with a pool cover at night
Insulate exposed pipes and equipment
Lower the pool water level slightly to reduce evaporation
Use a windbreak around the pool area
Keep the filter and heater maintained
Set the thermostat to the desired temperature