Don't try to fix everything with Photoshop. It's not just about adding some sky, since the result will always look a bit unnatural.
If you take a picture on a sunny day with a nice blue sky and a few clouds, then your whole scene will look different compared to a scene shot on a cloudy day, late or early in the morning. The contrast, darkness/length of the shadows, haze, color, etc, they all will be different.
See, it's simple to remove the white sky and replace it with a some nice blue sky with a few clouds that you found on the Internet, but it will only correct the sky and not the whole scene and the result is nothing more but an improvisation.
Try it out, mask out the sky and place a nice sky layer underneath and look at the result; you'll notice what I mean.
Btw, I had to go back to edit my post and couldn't find the edit button... \:]
Then I discovered that your picture was 2000 pixels wide! [shocked]
I don't know about any limits on this board, but 2000 pixels is just wayyyy to wide for a board like this, because I don't think that our members love scrolling left and right to be able to read every post in this thread.
Just my two cents
Need my sky back.... 
Linear Mode
