The method I used to replace the sky involves a mask based on the image itself. I think knowing how to make and use image-based masks is one of the most useful things you can learn. Here's a link to a basic tutorial in case you're not familiar with it:
http://av.adobe.com/russellbrown/AdvancedMasking.mov
So the first thing I did was roughly select the sky (although I was a bit more careful along the tent edge because it's close to the same color as the sky.) The rough selection looked like this:
http://home.comcast.net/~zumbari/Ret..._selection.jpg
Then I created an initial mask based on the luminance of that selection. It looks like this:
http://home.comcast.net/~zumbari/Ret...itial_mask.jpg
Black is opaque, white is transparent, and grey is in-between. The eventual intent is for this mask to be applied to my new sky. Since both the sky and trees are still grey in this mask, the new sky will partially show through both the trees and old sky. I want the trees to completely block the new sky and the old sky to completely disappear and let the new sky show through. So I apply a curve to this mask to enhance the contrast. It now looks like this:
http://home.comcast.net/~zumbari/Ret...with_curve.jpg
It's pretty clear how this mask will work--the black will block the new sky (thus making the original image visible) and the white will be where the new sky shows. Here's what the new sky looks like:
http://home.comcast.net/~zumbari/Ret...p4_new_sky.jpg
Here's how it looks after the mask is applied to it:
http://home.comcast.net/~zumbari/Ret...sky_masked.jpg
Now turn on the original image under the masked sky and you're just about done:
http://home.comcast.net/~zumbari/Ret...d_original.jpg
I apply my repaired tent pole and enhanced handwriting:
http://home.comcast.net/~zumbari/Ret...nd_writing.jpg
Viola! Well, not quite. I'm leaving out some brightness adjustments here and there I did to the sky, but that's a separate topic. I showed these steps because it shows what the mask looks like and how it behaves and I'm not sure what level you're at with masks. However, when I actually did it, I applied the initial mask to the sky and had the original image turned on underneath that first. THEN I adjusted the curve on the mask so I could see my final result changing in real time.
The new sky is actually the easy part (just takes a few minutes). The cloning and scratch-removing is actually far more time-consuming. The cool thing here is once you have the mask, you can drop in another sky almost instantly. Here's a color sky for fun:
http://home.comcast.net/~zumbari/Ret...ferent_sky.jpg
Bart