Ive found a couple, but they seem to be more useful for closer up shots. Still, they show how to make some nifty cloud brushes, and how to paint clouds from scratch. I may use the video tuts for my town maps. Most promising is the jpeg tut. I'm going to try out this method and get back to you guys with the results.
Here are all the links:
Making a wonderfully flexible cloud brush
Painting clouds into scenes from scratch
Original tutorial the jpeg was excerpted from