The single most important way to prevent freeze damage is to let the pump run.

Normally the pump is turned on and off automatically (or manually).  If the pump is turned off and the temperature drops below freezing, the pipes aboveground and pool equipment (especially heaters) will quickly freeze.  If the pump is left running the water in the pipes and pool equipment cannot freeze.  This is due to the water temperature of the pool just a few feet below the surface is always above freezing.  Some customers have been know to leave the pump run all winter with no freeze damage.