The Meaning of Life, the Universe, and Everything.
Since the old method of making custom player heads from any site broke in Minecraft 1.8.4, I've made a small tool that helps making them in newer versions. You will still have to change your skin in the process, but once that's done the heads will always remain the way they were when you generated the command, regardless of skin changes.
Make sure you have a backup of your skin. If you don’t, get one by downloading your skin file here, for instance.