This technique simulates the effect of split toning: combining warm and cool tones in the same image.
My original photo looks like this.
Press Command-Option-~ (the Tilde key, right above the Tab key) (PC: Control-Alt-~) to load the highlights in the photograph.
If the highlights keyboard shortcut doesn't work in Mac OS X Tiger, you'll have to go to System Preferences>Keyboard & Mouse and turn off the shortcut for Move Focus to Window Drawer.
Click on the Create New Adjustment Layer icon at the bottom of the Layers palette to add a Color Balance adjustment layer and in the resulting dialog, click on the Highlights radio button. Move the color sliders to change the color of the highlight areas.
Click on the Midtones radio button and move the color sliders to alter the color of the midtones of the photo, using similar colors to the highlights. Click OK when you are done.
Click back on your Background layer, and from the Select menu, choose Color Range and pick Shadows from the Select pop-up menu.
You'll get slightly different results if you use the "load highlights" shortcut and press Command-Shift-I (PC: Control-Shift-I) to Inverse the selection.
Add another Color Balance adjustment layer and click on the Shadows radio button. Move the color sliders to change the color of the shadow areas.
Click on the Midtones radio button and move the color sliders to alter the color of the midtones of the photo, using similar colors to the shadows.
Here's the finished product.
In this example, I added a Channel Mixer adjustment layer (below the other two adjustment layers) to make the image monochrome and make the warm and cool toning more apparent.
Variation 1: Channel Mixer adjustment layer added
Instead of starting with a selection of the highlights and shadows, this version uses two Hue/Saturation adjustment layers and Blending Options.
Variation 2: Hue/Saturation adjustment layers
Adobe Lightroom has its own built-in option called Split Toning, as shown here.