I drain my compressor after every use, or if I'm busy with a project, every night. I took out the crummy, hard to reach valve on the bottom and put in an elbow, a short length of pipe and a 1/4 turn ball valve. The pipe places the valve just beyond the edge of the tank, so it's easy to reach. When I'm ready to drain the water, a quick turn of the valve and the water is drained. IF the compressor is going to be turned off, I just leave the valve open.
Note that the air rushing out of the ippe and valve really loud! I actually make certain to have ear muffs on...
Also, if I just leave it open, towards the end of the draining process, the air rushing out stops, hisses, and then goes again, and stops again. Took me a while to figure out why.
As the air expands from 150 PSI in the tank to the outside, it cools, and as it's rushing out and cooling, it actually forms ice in the drain pipe. So, as it spits and pops, bits of ice are forming, impeding air flow, then being forced out. I never get much water, perhaps a couple of ounces from the bottom of the tank blasts out on the initial opening of the valve, but the remaining vapor condenses combines with the tiny bit of water left and freezes...darndest thing...
By the way, I bought the compressor in 2002. It gets used several times a month.