6 things to check your pool for to prepare for freezing temperatures

This article refers to swimming pools that are not drained for the winter and continue to run all year long. Basically, the first 2 points that need to be checked only once initially and then the last four checked weekly.
Make sure the freeze guard is working properly. There is a copper coil usually found coming out of the box on the bottom (some freeze guards have the coil inside the front cover, on these units be careful not to touch any live wires). Using plastic thongs, place an ice cube inside the coil or place next to it and wait at least five minutes. The freeze guard should turn the pool pump on. Moving water will not freeze.

1. Check all valves on the return side of the system and make sure they are fully open (these are located after the heater or if there is no heater, after the filter) ... also, if you have a chlorinator be sure to open the flow valve completely. One exception to this is if a valve controls a fountain or sprayer (you do not want water spraying all over the outside of the spa/pool or deck). Another exception is if you have a pool/spa combination do not open the spa drain valve (obviously, it will drain the spa).
2. Make sure the water level in the pool is in the middle of the tile (enough water to flow over the skimmer throat).
3. Keep skimmer baskets clean. Enough water passing through for good flow.
4. Keep pump basket clean. Look through the clear lid in the pool pump, it doesn't take much to interfere with the water flowing through the pump on some models.
5. Keep an eye on your filter pressure. Depending on the normal operating pressure for your system, high pressure will cause the water flow to slow down increasing  the chance of freezing.

***One last thing to mention is an extra precaution for nights where the temperature can get below 20 degrees or in the event of snow or ice is to place a plastic tarp over the equipment. Place the tarp high enough to give plenty of room to also place a drop light underneath it. It is very important to place the light away from any plastic pipes and tarp or any combustible material. I prefer the lights with clamps on them but don't recommend clamping it to a pvc pipe. The light will provide added warmth inside the tarp to help insulate your system.