Maybe you could add a special watermark, like your playername or some Symbols on your back, which even when copied, and they would still resemble you. (Unless the stealers are really too stupid to edit the skin, even if so, you would know that you're either the only one with that skin (w/ trademark/name). And if they edit your name awax, you still would know you're the one who wears the original
Or like SylverKitten said, if only YOU want to see your Skin, use a TexturePack (ResourcePack as it's called now)
