Odds are if you surf in the celebrity circles you've come across images that some pervert, like myself, has "X-Rayed." Some are good, some are really bad. Personally i consider it artistic, and it can be quite fun... but it's still perverted.
Your Personal "X-Ray" Tutorial: (procedure was written for Photoshop users)
Step 1 - selecting your photo
You will need a decent photo. Sheer is not the only characteristic the photo must posses, but it is an important one. Black sheer, in my experience, will not work, at least not nearly as good, but some people have been able to get some decent results.
The material itself doesn't really matter as you can see from the examples below, as long as it's sheer and a light colored material, you should be fine. Remember, you are exposing the darker/lighter tones from the flesh underneath the material... so make sure you have something to work with.
One more thing; You will notice that most people,(like myself), have prominently "x-rayed" the chest area, this is because the clothing is pressed tight against the flesh. Areas like folds or empty spaces will have much less to zero results.
Step 2 - selecting the area to "X-Ray"
Try to only select the area in which you are editing, don't do the entire image or it will look terrible. For this you will need to use the "Magnetic Lasso" tool and outline the specified area. IE; if you're "x-raying" the chest area, select the entire top with the magnetic lasso... not just the tits.
Once the area has been selected, right click within the outlined area and choose "Layer Via Copy." This will make a new layer from the selected portion. You may also choose to "Feather" the outline... but it's not necessary.
Step 3 - Obtaining "X-Ray" effect
Next, go to "Image" --> "Adjustments" --> "Levels." From there you can play with the slide bar to achieve the best effect. I can't really explain this part further as every photo is different, especially light and/or dark clothing.
From here you can start to get artistic, play with other levels like, colour, contrast, brightness, and most importantly the "Dodge" and "Burn" tools to perfect it. But be fore-warned!... do not become dependent on the dodge and burn tools... you will ultimately ruin the image by "drawing" on it. Only use it to gently darken the shadows that are already there.
That's pretty much it. As i said, some photo's will work better than others. It has a lot to do the the lighting within the photo itself as well. Yellow lights or natural light will create different outcomes, as will the clothing... Here are some examples that i've done: