We'll definitely do this one as an action, because you'll want to use it often, and the step-by-step method is delicate and somewhat lengthy.  So open an image you'd like to enhance, go to the Actions Palette, and click the "Create New Action" icon down at the bottom.  Name the new action "Adding Punch II" and click OK.  Press Ctrl-J to create a duplicate layer.  From the Image menu select "Mode" and then "Lab Color."  Important: when Photoshop asks whether to flatten the image, click the third button, labeled "Don't Flatten."

From the Layers menu select "New Adjustment Layer" and then "Curves..."  In the "New Layer" dialog just click OK.  Now comes the delicate part.

In the Curves dialog, note that the Lightness Channel is selected (which governs bright-dark interplay).

Select the lowest point on the curve.  The Input and Output boxes appear, with 0 showing in each.  Click in the Input box to select its 0, and change it to 2 (see left).  Then select the highest point on the curve.  The Input and Output boxes appear, with 100 showing in each.  Click in the Input box to select its 100, and change it 98.  The resulting curve has a slightly greater slope, thus compressing the bright-dark range and thereby increasing contrast.

Use the Channel selection drop-down box at the top of the Curves dialog to select Channel A (which governs the red-green interplay).

Select the lowest point on the curve.  The Input and Output boxes appear, with -128 showing in each.  Click in the Input box to select its -128, and change it to -118.  Then select the highest point on the curve.  The Input and Output boxes appear, with 127 showing in each.  Click in the Input box to select its 127, and change it to 117.  The resulting curve has a slightly greater slope, thus compressing the red-green tonal range, but it is symmetric about the center point to avoid introducing a color-cast.

Use the Channel selection drop-down box at the top of the Curves dialog to select Channel B (which governs the blue-yellow interplay).

Use the same technique as you did for Channel A to adjust the lowest and highest points on the curve.  The resulting curve has a slightly greater slope, thus compressing the blue-yellow tonal range, but it is symmetric about the center point to avoid introducing a color-cast.

Now press Ctrl-E to merge down the Adjustment Layer.  From the Image menu select "Mode" and then "RGB Color."  As before, when Photoshop asks whether to flatten the image, select "Don't Flatten."  Finally, press Ctrl-E once more to merge down to the original layer, and from the Actions Palette stop the recording.

All of this works because the Channels represent interplay between contrasting colors, and our eyes judge any color as being more vibrant when it's placed right next to its contrasting color.

On the right is an example from Alex Walker's article.  You can see the apparent increase in vividness when the red and green are placed in close proximity

Let's take a look at the results.  Below you see (1) the original image, (2) the image after applying our first Adding Punch action, (3) the image after applying the current Adding Punch II action, and (4) the image after applying Kevin Kubota's "Daily Multi-Vitamin" action:

All three actions add some punch to the image, as you can see, but in varying degrees.  To reduce the effect, you can either step backwards using Ctrl-Alt-Z to a point where you can decrease the opacity of the layers; or more simply, press Ctrl-J before beginning the action and then decrease the opacity of this layer when the action is completed.  To increase the effect, simply run the action a second time.

To make life simple for SONOPP folks, who rarely have enough time to play around with creating actions, you can download both the Adding Punch and Adding Punch II actions by simply clicking here.  (Sorry, can't give you Kevin's action.  You can buy it, though, along with a host of others; it's in his "Artistic Actions Vol II v2.3".  For a commentary about Kevin's actions, see becker's blog.)

(P.S.: Madison and Pearls furnished courtesy of Grandma Linda.)