Painting - Painting clouds

I'd like to paint my son's bedroom walls like the sky. What is the best way to make it look realistic? Do you know any sites that have online instructions for painting a sky mural? Any suggestions would be appreciated.

It is so simple you wouldn't believe it. I did it on several walls. This is how you do it: Make sure you start with a clean palete (white), this way you get the blue you want. I rolled on the blue, do this in small segments. While it is still wet using a natural sponge torn in half dip it into white paint and sponge it on the wall. The wet paint becomes the shadows in the clouds. See, simple! The hard part is doing it on the ceiling. Its hard on your back, so be careful.