I've read that you need to drain air compressors after each use because condensation can rust out the tank. The problem is, the existing drain valve (see above) was 1) difficult to turn 2)leaked air 3) broke twice.
So I googled "air compressor drain valve" and found this post outlining how to upgrade it.
Next, I went to Emigh hardware and got the parts for about $24.
I used to have to tip the tank over to get access to the drain valve too, now it sticks out the side. Pretty sweet.