Skies are also something I need to work on. Often when I'm out, I wish I had a camera so I could take photos of some of the amazing clouds and skies I see to practice on.
Anyways, this isn't a tutorial but a tool to help you with sketching. I just discovered it and haven't tried it yet, but I think I'm gonna try and make it part of my daily routine :
http://www.posemaniacs.com/?pagename=thirtysecondThe whole site seems to be full of awesome reference, I think it's great!