Are you using the 'fill' tool? If so, don't. I used to do that alll the time when I drew a picture, but now, I use layers. They're really nifty when it comes to seperating your background from your colour and lineart. And your colour, from your lineart and shading etc.
Just post an article in the forums and I'm sure someone will be happy to help.

Besides that, I think that this is very nice :)