I never drain at all. I install a screw in device called a Gizmo into the threaded hole in the bottom of the skimmers. These are plastic tubes that extend up above the water level and are about 3" wide in the middle. There is an opening at the top of the tube which you can use to blow the lines out, and then a small cap to close it with. The water stays in the skimmer and when it freezes this tube absorbs the expansion so there is no damage to the skimmer. They cost about $10 or $15.