The two main parts of this tutorial include:

- duplicating a channel to use as a source for selections

- using adjustment layers for dodging and burning

The tutorial is intermediate level, but only because it uses adjustment layers.  Otherwise, if you can paint by numbers, then you can do it.

Here's a picture of a woman someone posted. We'll call this the 'base scan'. 

The poster asked how to make her skin 'wet'.  So that's the goal, to make her skin 'shimmer'.

The first thing to do is fix the scan.  She has a typical Bunny Body, with a fairly nice figure so we can leave the shape alone. If she wants any more enhancements she'll have to pay the plastic surgeon or go back to the gym to lose the extra 3 pounds of body fat she's carrying around. So while she's not flat, the picture is, so the first thing you want to do is fix the scan. Use whatever methods you are comfortable with, but it's a good habit to try and use global corrections. For example, it's best to do just a levels (or curves) correction, as opposed to a big convoluted recipe of corrections, such as a levels correction, followed by a hue/sat correction, followed by a colour balance correction, and finally, a brightness/contrast correction.

Instead, you want to get a lot done in one stroke. The more you do, the more you will wreck the image. If you don't know how to do a levels correction, there are plenty of tutorials on the net for this and it's definitely worth your while to learn how to do a levels correction on a scan, as it's a basic skill and it's fairly easy.

So, our objective is to make her skin appear shimmering, almost wet. Since the picture is flat-looking, we need to boost areas of contrast. If you look closely at her... okay, refocus and start over...

If you look closely at her you'll see that there are parts of her skin that are already lighter (decent highlights) and parts of her skin that are already dark. This is great.  And, if you've done the levels correction then you'll see an improvement from the base scan.  But to get the effect we're looking for you'll want to boost the highlights more, and darken the other areas as well. This contrast will help give the illusion of 'wet' skin. 

At this point I'd recommend making whatever selections you might need.  This is optional, but still worth the time.  For this picture you'd want to at least make a loose selection of the highlights and shadows.  A good way to select both of these areas simultaneously is to look at each channel and pick the one with the best contrast, then duplicate it in the channels palette and then, using curves or levels, exaggerate the contrast in it.  This channel will act as a selection for the 'lights' of her skin, and, a selection for the 'darks' of her skin.  The blue channel is the best to use for this image.  So go into the channels palette and drag the blue channel down to the upturned page thingie at the bottom of the channels palette and this will create a duplicate.  Now hit Ctl+M to bring up the curves dialogue box and steepen the curve.  See here:

Once you've done that, blur this duplicated blue channel by several pixels.  It doesn't have to be perfect, as it's only going to be used as a source for selections.  You might need it later.  (If the above procedure is too complicated, just skip it. This is just a fancy way to make a selection.)

Return to the layers palette.  Here's the big trick.  If you learn this you can dump your dodge & burn tools forever.  Duplicate the background layer.  (This isn't absolutely necessary but I do it as a habit.) You're going to make an adjustment layer on the duplicate layer.  To do this, select all, then click on the adjustment layer button at the bottom of the layers palette.  Choose curves from the drop down list.  See pic:

When the curves dialogue box opens, tug the center of the curve down to make the picture considerably lighter and click ok.  Now you'll have an adjustment layer, it will appear as a layer above the background layer and duplicate of the background layer in the layers palette.  The problem is that now the picture is too bright.  What you're going to do to fix that is invert the adjustment layer by hitting Ctrl+I, and once that's done you'll notice that the picture has returned to its old self, as the effect of the adjustment layer is now hidden.  (There are other ways to do that but nevermind, just invert the adjustment layer.) 

Here's the cool thing - in a moment you'll begin to paint, or use gradients to reapply the effects that are hidden in the adjustment layer.  The effect that you made by creating an adjustment layer to make the picture brighter are still there, only you can't see the effect right now. 

But before doing that, to be fair, we should make another adjustment layer for the darks.  So repeat the process, ie. return to the duplicate background layer, click it to make it active, select all, click on the adjustment layer button, choose curves, but this time tug the curve upwards to darken the image.  (Incidentally, once you're in the curves dialogue box you can also manipulate the colours individually if you wanted to, and you will once you're comfortable with this process.)  Once you're satisfied, click ok.  Again, repeat what you did with the other adjustment layer, by clicking Ctl+I to invert this adjustment layer as well and temporarily hide the effect.  Your layers should look like the picture below - I've named one adjustment layer 'lights' and the other 'darks', but that's just to help you understand.  You can name them anything you like, or don't bother naming them because that's just a waste of time.

Here we go: click on the 'lights' adjustment layer to make it active, grab a small, soft brush (or a gradient tool set to fade to transparency), lower the opacity of the brush (or gradient tool) and with your colours set to default black & white on the main toolbar, with white forward, begin painting with the brush in the areas of the skins' highlights.  If you've never done this before it'll probably freak you out.  What you're essentially doing here is dodging using an adjustment layer.  What's even more amazing is you can filter this, smudge it, or do many other things.  It's a very flexible technique.  If you make a mistake, or want to make a change and reverse yourself, then just click the toolbox colours to bring black forward/white behind (or use hotkeys if you know them) and paint 'out' the effect.  In other words, if you paint with the default colour at 'white' you'll be painting in the contents of the adjustment layer, and if you paint with the default colour at 'black' you'll be painting out the effects of the adjustment layer.  You can go back and forth, back and forth, painting in with white, out with black, until you're satisfied.  Also, as I said, you can smudge it, filter it (blur is good here).

Repeat the exact same steps for the dark adjustment layer that you just did with the light adjustment layer. Only here you'll be burning using an adjustment layer.

Incidentally, you can do this with any kind of adjustment layer, not just curves.

You'll eventually end up with something that has caused the highlights to pop out, and the shadows to darken. The rest is applying effects and corrections to make it look the way you want it to, and one of the ways you can do that is by going into the channels palette and activating the selections from the duplicated, contrasty, blurred blue channel that you made at the beginning.  You might want to play with some blend modes or add grain to her skin, for example.

Have fun,

Mig