People choose skins because they make their computers look the way they want them to. When you're looking at a screen for eight hours a day, that's worth a lot, so it's no surprise skinning is popular. But even the most satisfied user probably wishes there was something different about the skins they use - perhaps the titlebar could be a slightly different shade of grey, or they'd prefer it if a button performed a different action to the one you specified. Sometimes a little more effort on your part can make their desires possible, even easy.
Use options like colour-masking, configurable buttons and similar optional features to enhance the flexibility of your skin, so making it appeal to a wider range of users. If the format you are using does not provide the flexibility you or your users desire - most often because there is no support for re-colouring - consider offering different versions of your skin.
If you think users are likely to try modifying your skin on their own, adding comments can often help them and save you the time of explaining it more than once. They can also help you when you come back to it six months later and try to figure out what you did! Also, consider writing some documentation; how to apply the skin might be a good start, but at a minimum, include a readme file giving your website and/or contact address - if you want nice comments, it's important to let people know how to get in touch with you!
Quick Hint: If you got others to test your skin before release, you can add customisation features while waiting for their comments.