Presented here is an interesting technique for retouching a model laying on a seamless background. The following five images will take you through the steps. First, retouch the model to your liking while disregarding the grey seamless:
This next step is where all the work is done: mask out the grey background. Once you have done this, you will want a layer of the model without the mask. To achieve this, in the Layers Panel, click on the thumbnail of the retouched layer to make sure it is selected. Photoshop will place a little frame around the thumbnail. Next, Command (Control) click on the layer mask to get the "marching ants" showing the that the model is selected. Do a Command-J (Control-J) to bring the bring up the model onto a new layer - with no layer mask.
This is done so that the selection of the model can be defringed by one pixel (Layer -> Matting
-> Defringe...). This helps to remove unwanted remnants of the grey background. Next, place a new layer beneath this layer, and fill it with white:
A new blank layer is created above the white layer, and this layer is filled with pure black. A white layer mask was added to this black layer. Using the Elliptical Marquee Tool, a horizontal oval was drawn on black layer, and the resulting selection was given a feather of 800 pixels (Select -> Modify -> Feather... 800 -> OK). Try a feather of 150 pixels if your image was taken with a camera with a 12 MP sensor. When this feathered selection of the layer mask is filled with black, the image should look like this:
Next click on the top layer of the model (without the layer mask) to activate it. Click on the Layer Styles icon at the bottom of the Layers Panel. Select Drop Shadow... and create a drop shadow. Adjust the Angle, Distance, Spread, and Size to your liking, then adjust the Opacity. I unchecked the Use Global Light checkbox. Hit OK.
The drop shadow showed in a few places along the top edge of Andrelica where I did not want it. I Control (right) clicked on the Drop Shadow in the Layers Panel and chose Create Layer from the pop up menu. I added a white layer mask and painted away any unwanted drop shadow with a black Brush:
One could stop here, but color can be added. A Hue/Saturation adjustment layer was added above the black layer. The Hue was set to 0 and the Colorize box was checked. This yielded the final result: