Shoutbox

I'm not a skinner, but I have wrote some scripts in the past. It's not exactly the same, but I'll drop my opinion here anyway.

Back when I used to script actively, I regularly took a look at the source code of other interesting scripts. Not to copy it directly, but to learn how certain problems could be dealt with efficiently.
One of the things I noticed was that most scripters put a comment on top of each script file saying that if you want to use any code, you'll have to contact them through this@email.com or put their names in the about window or some place else.
I personally couldn't care less about people using my code for whatever they think they can use it for, so I put a comment on top saying "do whatever you want with it"

As I said, I am not a skinner. However, I can imagine that you could do something like that with skinning too.
It doesn't prevent copying, but it does remind people to contact you if they want to use anything from your skin.