This is new to me but I apologize if old hat to others. Want an easy way to replace white or drab sky even among trees and around objects without making a selection, then try this.
Open image with white or dull sky, Open same size image with good sky. Alternative is open blank image of same size and pixels as original, add gradient and or render clouds to make your own sky.
Copy and paste or drag the original bad sky image into the good sky image as a new layer. Bad sky image should be top layer.
Change mode on upper layer (bad sky) to darken.
Double click lower layer (good sky) to change from BG layer to full layer, dial down opacity to suit.
If there are any details from the good sky image that are showing through that you don't want then make this layer active and paint out the unwanted details with white.
The same for the original image in that if there are grey or dark areas blocking the new sky then paint them out with white on the original layer.
Move the good sky layer around if necessary to just cover the sky areas.
Seems to work great on some images and so so on others depending on lightness and darkness. But when it works it is a time saver.

Thanks to Tim on the Jasc users forum for this tip.

CaseyJ