Wave a (Photoshop Elements) Magic Wand
Let's get started. I'll work with this photo--the sky is uniformly bland, and there's enough of it for you to see what's going on. We need to select the sky but exclude the skyline; that's a job for Adobe Photoshop Elements' Magic Wand. Select the tool, and then open the Options palette and set the Tolerance to about 15. Click the sky, and the tool will select all nearby pixels that share a similar color. If you need to add bits of sky to your selection, choose Add to Selection
and then click more of the sky. To remove a little, click Subtract from Selection
, lower the Tolerance, and click the unwanted part.