After I completed the shot I was told that the woman looks too flat and I need to add depth to make it seem like she isn't just one layer. Any ideas or techniques to give the illusion of depth to a 2D image would be appreciated.
It's going to be tough to do if all you have is a 2D image. I don't know who told you she looks too flat, either a client, or your boss/producer/etc., but they need to understand that if shooting a real, or adding a true 3D woman to the project, isn't in the budget, then looking 3D isn't much of a possibility.
You might want to post a still of what you're doing, which would help us figure out what the look is. Is the "looks to flat" comment mean that there isn't any dimension to her (which of course there isn't, or do they mean that the lighting of the woman in the shot makes her look flat and 2 dimensional? You might be able to add some shadowing in Photoshop to make her look more 3D, but that's a different issue. My guess is that they mean the effect. Please show us a still of what you're doing.
There are all sorts of ways to make it look different. Instead of using the Linear Wipe like Andrew says, you can use a solid and animate a mask to make the transition curvy and changeable. It ought to look better, especially if you can make that animated mask kind of follow the contours of the subject you have in mind.
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA