And since King of Photoshop insists I fake my shit: randomwoman1.psd
The nipples are certainly there, it's just a matter of bringing the definition back to them.
http://www.metacafe.com/watch/400249/photoshop_x_ray_trick_see_through_clothes/
That was the first video I watched on it, but if you have a program, I can tell you some basic steps.
First, you want to select the area that will be X-Rayed, using the Magnetic Lasso/what have you.
Then, go to (in my version of Photoshop) Enhance>Adjust Lighting > Levels
For light clothes, you want to Bring the Third arrow down, and bring out the first two arrows in just the right way to enhance what's underneath. I got the hang of it by tweaking around with google image search photos of people in white shirts, etc.
I don't have a firm enough grip on the subject to really give more advice other than the fact that many, many pictures will not fit the criteria of light, thin, and tight, enough to the point of having a successful shop.