Notice how the bright flowers are unaffected by the saturation boost, while the green background and the rest of the image gets more saturated.

I have also made it into an action which you can download at the end of the page.

Step 1:

Make a Gradient map adjustment layer with a gradient from black to white, to get a black and white image. Now set it to difference, select all (Ctrl+A) and copy merged (Ctrl+Shift+C). You can now delete the gradient map.

Step 2:

Enter the channels panel and make a new alpha channel, and paste (Ctrl+V) .

What you have now is a channel where the white parts represent the areas with high color saturation and the black parts the areas with low saturation. By using this as a mask, you can boost the saturation only in the areas where it’s needed the most and also avoid the artifacts you can get when just slapping on lots of saturation. But first you need to invert the mask. Also, you need to increase the contrast of the mask with levels to differentiate the effect more between the high and low saturated areas.

Step 3:

Then Ctrl-click the channel to make it into a selection, and make a Hue/Saturation layer.

You can now increase the saturation a whole lot more than usual before it looks bad, because it specifically targets those areas that are low on saturation.

Something else that is kind of nice is to turn the saturation on this masked layer down to a negative value. All the way down you will get a selective colored image where only the most saturated colors show, the rest being black and white. Turn it back up a bit and you can get a nice effect with a desaturated image where the most saturated parts are still very colorful.

Here is an action which does all the work for you DK_Vibrance.atn. It automatically maximizes the contrast of the alpha channel, and leaves you with a Hue/Saturation layer that you can tweak the saturation of until you are satisfied.

It works with PS CS and later, might work with earlier versions too although I have not tried it.