This is a relatively difficult topic.
I will just make some, hopefully, useful introductory comments and sketched the idea.
This image here, which is the Singapore Esplanade Theatre, mirrored and flipped all over, was processed in two parts, namely separately for the sky, and for the spiky other part.
There are several ways of doing such selective processing.
One of this is to mask the sky, or foreground. And then there are also several ways to create the mask.
The easiest in this instance was to create a mask from the sky, and again there are also several ways of doing that.
And one way is what I have called "Natural Masks".
If you click the Channels tab in Photoshop's Layer Panel, you will see four images, namely the image as depicted by the Red, Green, Blue (R,G and B) channels, and the composite, combined RGB channels. Alpha channels are other than the RGB and composite channels.
In this instance the Blue channel will be almost completely white in the sky area, and completely black in the spiky foreground, ie you already have a ready-made mask for the sky in the blue channel.
In other words the blue channel is a natural mask for the sky, which you can extract "naturally", without use of the magic wand, lasso, pen tool, etc.
To extract it, you copy the blue channel - select it in the Channels panel, Hit Ctrl-A, and then Ctrl-C - and go over to the layer panel, Alt-Click on the intended mask, and then click Ctrl-V.
(You can also hit Ctrl-3 to select the Blue Channel. Ctrl-1 and Ctrl-2 selects the Red and Green Channels respectively.)
Now this is of course too simplistic.
For the thing that makes a good or bad mask is in its edges. In this instance the edges have to be hard, and not fuzzy.
The are again several ways to treat edges, but a good start is Photoshop's Refine Edges in the Selection menu. This feature is only available with CS3.
I have to stop here for the introduction.
The best thing now for you, is to go fiddle around with this idea of natural masks.
There are lots you can do from extracted natural masks.
For example the red channel can make some image really go pop when used as a mask for a Softlight blending layer.
And then there are Calculations.
But really I have to stop here. :-)
