There isn't much to this other than the amount of time you spend to get the look you're after. This tutorial is only intended to give you a visual overview of what to do in Photoshop.
1. Open your image in Photoshop. I'm using one here that was submitted to the photo retouching forum by auroraskye - http://www.retouchpro.com/forums/photo-retouching/16565-fashion-retouch.html
2. You're going to create 3 curve adjustment layers.
The layer named Dodge is a simple curves adjustment to make the whole image significantly lighter.
The layer named Burn is a simple curves adjustment to make the whole image significantly darker.
When you first make these adjustment layers they already have a mask attached to them but the mask is white. You want to select the mask and fill it with black. This will 'hide all' of that adjustment layer. You wil be painting on the black adjustment layer with a white paint brush to reveal you lighter and darker adjustment layers. If you shift+click on the layer mask icon you can disable it and you will see a red X on the layer mask in the layers palette - in my screenshots I'm using Photoshop CS3 beta but this technique works in the older versions as well.
The 3rd layer you'll want to create is a contrast boosting layer so you can more easily see the variations you want to get rid of. It's a curve adjustment layer just as the other 2 but this time we're going to make an S shaped curve. Adjust to whatever will work best for the image you are working on. You won't need to change the layer mask to black on this layer, it's simply a filter to help us see the tonal variations better.
That's all there is to setting it up. All that's left is to paint, paint, paint and paint until your arm falls off or your eyes start bleeding. This is probably not the best way to tackle what I have for an example image because of the amount of time you'll end up spending doing tiny strokes all over the place but I felt this was great for an example.
I started by working on my Dodge layer mask using a 3 to 10 pixel, soft edged brush set at around 13% flow. I use a Wacom tablet so these settings may not be the best for a mouse but as a general rule of thumb it's MUCH better to have to hit a spot 4 times to get it painted in nicely rather than hit it once or twice and have it painted in too much. This is what I ended up with for my Dodge layer mask.
For my Burn layer on this image I used a soft brush at 20 pixels and a flow of 7%.
Looking at the masks like this may help some. To view the mask all by itself you alt+click (option+click on Mac) on the layer mask icon in the layers palette. Normally when you are working/painting with white on the mask to reveal you don't see the mask, just the image under the adjustment layers. Here's a screenshot with all of my layers turned on after I spent hours painting away... including the contrast boosting S layer.
There is still alot of work to be done if you wanted it to be perfect, which perhaps is why more people don't take the time to walk through something like this! Then again, maybe I don't understand this technique all that well and someone else will come along and correct me :)
This is the result of the above, move your mouse over it :)