BACK TO INDEX OF PHOTOSHOP TIPS

This technique requires an understanding of "layer masking". If the image and palette to the right make perfect sense to you then you can jump to the next section. OTHERWISE read on...

This example is admittedly unsubtle but it is an easy way to learn how masking works. Looking at the layers palette, note that the image is on the bottom, the so-called background layer, and there is a Photo Filter layer above that is adjusted to convert the image to red. The small box to the right side in the Photo Filter layer represents the mask. I have painted onto the mask by first activating the mask by clicking on that box. (It will take on a double border when it is active. Then paint on the mask by painting on the image with the Brush tool (foreground color must be set to black and, in this example, set to 100% opacity and flow). To correct mistakes, paint with white by changing the foreground color to white. (You can toggle between black and white with the D and X keys.
The Photo Filter layer adjustment affects everything in the image EXCEPT what is put on the mask. Think of it as a "not". Or think of it as a way to "erase" through the Layer Adjustment. Masks can contain some pretty complex stuff, more than just Merry Xmas. Generally, the Brush settings would be given some very mild settings to be able to make subtle alterations to the effects of the Layer Adjustment.

NOW TO THE GOOD STUFF
There are about 15 global image adjustments available as layers as well as any text layer or image layer that you could think up and a myriad of blending modes. Any of those can be modified with a mask. Generally for everyday photographic purposes, I would make a global adjustment with an Adjustment Layer ( such as Curves, Levels, Hue/Sat or PhotoFilter) in such a way that the most important parts of the image are placed where I would like them to be, then paint on the mask with weak black (by adjusting the brush parameters) to restore a few small areas that were adversely affected by the Layer Adjustment. If you go too far, reverse the background and foreground colors (x) and restore with white paint. The beauty of this method over doing similar adjustments with the Eraser tool or the History Brush, is that the Image, the Adjustment Layer and the Mask are all kept separate until the very end when the image is merged. The music equivalent to this is studio recording in "isolation" - if something screws up you can also redo that single track.