So, first of all we need an image to work with. Here’s one of my little cousin:

As you can see, the image appears very “flat”.  I know it’s not exactly a fantastic photo to begin with, especially as it’s from a 1mp camera phone, but it’s good enough to demonstrate this trick!

So, once you have your image in Photoshop, right click the background layer and select “Duplicate” to give us an exact copy as a second layer.
Then, choose “Filters->Other->High Pass” from the menus and when prompted, select a radius of 10 and click ok.
You should end up with something grey looking, similar to this:

Next we need to make our new layer an “overlay”. To do this, select the dropdown list of layer types and choose “Overlay”.
This next image shows the location of said dropdown:

Now we need to apply a mask to this layer, as we don’t want to apply the high pass results to the full image. So, whilst holding ALT, click the “Add Layer Mask” button, located at the bottom of the Layers pane.

Masks work by using the colours white and black to determine if whatever is being masked should pass through, or not, respectively. By holding ALT when creating our mask, it will be created entirely black (meaning none of the layer will pass through). This is what we want, as we only need to apply the high pass results to specific areas of the image, as you’ll see next!

Ok, now we have our mask, ensure it is selected. It will have a black and blue border around it if it’s selected – see the next two images for an example:

Now, select the paint brush tool, and the colour pure white, then start to paint over the eyes, hair, mouth and teeth (or any other areas you wish to “draw attention” to within your image).

Once you’re done, you should end up with something like this:

which, if you compare against the original, seems to pop out and his face draws your attention more:

Hope this is of some use