There's nothing like a clear blue sky in real life, but it doesn't create for the most interesting photos - Weblog MakeUseOf has a nice tutorial for easily creating clouds to spice up the skies.
There are a number of ways to do this in Photoshop, but MakeUseOf opts to use the very handy Render > Clouds tool (Under Filter), which creates a cloud-like pattern using your currently chosen foreground and background colours (which, as long as we're talking about Earth, will likely be white and blue, respectively). They might not be the most realistic clouds of all time, but you may be able to tweak the Levels a little bit to make it look as good as possible. Hit the link for more detailed instructions if you're not already a Photoshop guru - and if you are, let us know your cloud-creating tips in the comments!
Hot To Easily Create Clouds in Photoshop [MakeUseOf]