Such was the great feedback I got after
my HDRI tutorial, I thought it might be nice to post some more
Photoshop tips and tutorials from time to time. I am no expert by any means, just a keen amateur, but perhaps I might inspire you to take the plunge and start improving some of your pictures (or try at least). As always, click the images to see a larger version.
So today I thought I'd show you a very simple and easy way to replace the sky in one of your photographs. I'm sure this has happened to you. You take an otherwise great landscape photograph but mother nature has let you down by providing a very dull, flat, uninteresting sky. Using Photoshop CS4 (or your preferred application) you can easily replace this with another one you might have taken on a different day. Now before you start leaving comments, there are several ways of doing this. I'm showing you this way, because it is very easy. If you'd prefer something a little more in-depth and professional,
you might prefer this video.
I have a picture that I took last Winter of some rooftops. It is a perfect example of what I have been talking about. I open it in Photoshop along with a second image of a nice blue sky with some clouds that I took a few weeks ago. Using the
Move tool, drag the sky image and drop it on to your original picture. It will automatically create a second layer.

Now click on
Edit -> Free Transform and drag and position your sky until it covers the desired area. It is better to overlap with your landscape a little to ensure no gaps. Once you are happy, confirm it.

In your
Layers palette, choose
Multiply as your blending option. This will allow the original layer to show through a little. Now choose the
Eraser tool and pick a brush size of around 150px. Ensure the
Hardness level is set to 0. Carefully erase any overlap with your
horizon. I have done the right hand half of it in the picture below. You don't need to be 100% accurate but take your time.

You're nearly done now but there is one more step to add a little realism. As I am sure you are aware, a sky is always paler close to the horizon and the colour gradually deepens the higher you go. To re-create this in our picture we must ensure that our standard colours are reversed so we have white as a foreground and black as the background. If you don't the effect will be back to front.

Now use the
Gradient tool, and choose
Foreground to Transparent from the tool properties drop-down box.

Now a draw a vertical line in the middle of your picture, starting just below the horizon and stopping approximately three quarters of the way up your sky. You might need to experiment a little to get the right look. But once you're happy, you should get something like effect below. First the
before...

And now the
after...

I think you'll agree it improves things greatly. I hope this was of use to you, and that you enjoy experimenting.