I know a fella on another forum who's put together a couple of tutorials for map-building in Photoshop (it's a SF & F forum, and we all like to make our own maps). He's only got two up (building the map from scratch in PS, and then adding the paper background), but they're good:

http://www.jezelf.co.uk/tutorials.htm

Whilst the building a map from a sketch tutorial isn't yet written, I doubt it'd be hard to follow the steps from the first tutorial, but build it to a general shape, rather than randomly placing the cut out bits together.

The only criticism I have about your map is the jagged lines. Not that they're jagged, but that there are too many of them, and they're very sharp. Personally I think that fewer looks better.

You might want to put in a few rivers, too, and maybe mountains and forests, just to make it look more credible.

EDIT: If you need persuading that the tutorial works, here's one I did in Photoshop using it as a guideline:

http://i3.photobucket.com/albums/y97.../mapklomar.jpg

Notice the supreme lack of mountains a trees.