Here's my method with Photoshop. First off, forget using the burn tool, it will wash out the colors and definition of the nipples. Secondly, this method will get results more often (albeit, not all the time) even on thicker cloth like bikinis.

The biggest factors in getting results are: 1. Plenty of light happening at the time the picture was taken (so that the light penetrates the clothing more) 2. Using higher resolution photos (to get more definition) 3. The photo itself is not washed out (i.e. too dark or too bright to the point at which there is no color variation will remove definition)

Getting Underneath the Clothing 1. CTRL+L to open up the Level tool 2. Hold ALT and move the left slider to the right until color around the breasts starts to change. Continue holding ALT and move the left slider back left a little until those color changes are undone. (You want to get the left slider as far right as possible without destroying any color definition in the nipple area. When the color starts changing, it means definition is being lost. You can push it as far right as you want, but the more color definition you lose, the closer you get to removing the nipple's color variation.) 3. Do the same for the right slider, but towards the left this time. Sometimes the color starts to change immediately. In such a case, you can feel free to just leave it at its default location. 4. Apply the changes, then go to the channel mixer (ALT > I > A > X) which we'll use to bring out the definition of the nipples 5. Change "Preset" to "Black & White Infrared (RGB)" 6. Typically you want to start off here with the Red slider all the way to the right so that it's using all the Red it can in the photo (which is where the color definition for skin will be) 7. Also start with the Green and Blue sliders all the way to the right so that it's using all of those color definitions as well. 8. Now get comfortable moving the "Constant" slider left and right. Notice how it will modify the color definition as you're moving it. This is the slider you'll use to seek out the definition of the nipples. 9. Experiment first by moving either the Green or the Blue slider all the way to the left so that its color definition is removed from the photo. Then move the "Constant" slider back and forth. Typically, at this point you'll easily spot where the nipples are, but it will be very poor quality. This is because that slider you moved all the way to the left contributed color definition to the nipples, whether it was Green or Blue it doesn't matter. 10. So move that slider back to the right maybe a 1/4th. Then move the "Constant" slider back and forth until you get the nipple definition back. You'll notice that the quality is becoming better. 11. Experiment by now moving the other slider (Blue or Green, whichever is still all the way to the right) to the left maybe a 1/4th. Then move the "Constant" slider back and forth until you get the nipple definition back. It may be more defined now. 12. Keep experimenting with different values for both Green and Blue. Typically, I find that having one 1/3rd from the left and the other in the middle gets pretty good results. If you're feeling like it's not quite working, at this point you can try moving the Red around too. Ultimately, you're looking for definition and quality. Sometimes you have to compromise and go with more definition at the loss of some quality. 13. Once you have the definition of the nipples, hit OK and apply the changes. At this point, you'll want to make a copy of your layer and hide it for safe keeping.

Bringing Out the Definition of the Nipples 1. FORGET THE BURN TOOL 2. You want the "Elliptical Marquee Tool" which is an alternate of the dotted rectangle tool. (Shift + M until the correct one shows up) 3. You'll be working one areola at a time. Hold SHIFT and start at the left side of the areola. Move right until the width of the marquee tool is the same width as the areola. 4. Move the Selection over the center of the nipple. 5. If the areola is not a perfect circle, right click the Selection and click "Transform Selection". Then hold ALT and shrink or expand the height until it matches. If the areola is at an angle, rotate the Selection by placing the cursor outside the Selection first. Then hold ALT again and shrink or expand the height to fit. Once it's good, hit ENTER to accept the Selection transformation. 6. Now "Feather" the Selection: "Select > Modify > Feather..." or "SHIFT + F6". Decide a good outline width for blending the edge of the areola. For higher resolution photos above 1000 pixels in height or width, 6-8 should do. For lower resolution photos, 1-3 should do. 7. Now CTRL+SHIFT+C to copy it, and CTRL+V to paste it into a new layer. 8. The new layer should now be selected. Change the blending mode for the layer to "Multiply". (It should be under a dropdown that's currently selecting "Normal"). Now tone down the opacity until it starts to look natural. 9. Do Steps 2-8 for the nipple inside the areola. And lower the opacity until it's down to a natural color. 10. You should now have a defined nipple that is not washed out but has kept its color definition.

You can now use this modified graphic on top of the original photo with different blending modes and other Photoshop magic to bring the color back.

One thing to look out for: depending on the folds, seams, and shadows of the clothes, it might come together in such a way that it makes something appear like a nipple when in actuality it is not; so double check. Use common sense to locate where a nipple should logically be located.