I'm sure this is a pretty common thing to do when you have a dull sky but nice foreground.
So how do you replace the sky in Photoshop?![]()

I aggree I don't normally replace the sky, but I did a NT colour guide a few months back at work and it was a summer/stormy day, the image was of a front of a building bathed in sunshine (early morning/evening) but the sky was as black as the ace of spades, so I was asked to retouch in a nice blue sky.
Found the image on NT webiste, this is the original, but I'm not at work so can't show you the retouched version...
http://www.ntpl.org.uk/thumbnails/00000000142/71499.thw