As previously discussed, vignetting is a great way to make the portrait model "pop." By darkening the edges of an image, the viewer's eye is drawn the the brighter, more contrasty part of the image, which is the model. Examine this image of Andrelica. The top part of the image is too bright and too busy:
An easy way to vignette is to add a Curves adjustment layer, set its blending mode to Multiply (to darken everything), and then with the layer mask selected, paint over the model with a large, soft, black Brush. This reveals the model in the image's original state before the Curves layer darkened everything. Using this method, here is what you get:
The problem with this technique is that it can often leave an unsightly bright halo around the model, and further, it is difficult to darken tiny areas. Another solution is to go back to the layer used to edit the image, and use the Direct Selection Tool to select the undesirable bright areas in the background. In this example, the top part of the image was selected, as well as the bright triangular area under Andrelica's right knee. Once you are satisfied with the selection, add a Curves adjustment layer. Leave its blending mode set to Normal, but pull down the midpoint of the curve to darken the selected areas to your liking. Here is the result:
In conclusion, this is perhaps a more elegant way to vignette. Not only can you better control what gets darkened and what does not, but you can also control how dark you want these areas to be.