Let's answer some questions first.
Q: Can't I just use falsies?
A: Sure, but will look pretty awful from anything other than a decent distance. Either you'll have a plastic, two-tone chest, or you'll have a cave-like recess. People aren't built like that.
Q: Why the hell would I do this?
A: Assuming you don't make up the T in GLBT, you are probably either an actor who needs to pull off a convincing gender transformation, or you want to have the best Halloween costume ever.
Everybody ready? 'Cause here we go!
First, shave all the hair off your chest. That should be pretty obvious. Non-electric razors are best for that ultra-close shave. You should probably shave the whole area twice to make absolutely sure you're smooth as a baby's bottom. Be sure to wash yourself afterwards to prevent ingrown hair, zits and the like.
Now you'll need some tape. No duct tape. Medical tape works best, because you'll need something that will continue to stick to you, even if you're sweating. If the tape comes loose, you instantly loose half your rack. You need two pieces, one end will go just below your armpit, about on the same level as your nipple. The second end should reach to the opposite nipple.
Now, attach the end of the tape that goes under your armpits, angled so that it will pass just below the nipple on the near side. Now take the loose ends of the tape and tug them across your chest at the same time, bunching your flesh together, creating cleavage. This might hurt a little bit, but medical tape usually has some give to it, so it'll fit you.Stick the tape to yourself, and then stick one more piece of tape to tape down the first two pieces, just for a little extra security.
From here on out it's all about makeup. Now's a good time to put on your fake boobs, so you can be sure you're creating the proper effect. Use foundation cream to cover up any stubborn hairs or light stubble left over from shaving.
Use a darker foundation or eyeshadow to draw a line along the curve of the cleavage. To enhance this effect, brush a darker shade in the recess of the cleavage, and apply highlights to the rounded portion of your faux breasts. Remember to blend.
Now put on that low cut shirt and run amok.
When you take the tape off:
For crying out loud, this is not a band-aid. Do this slowly, carefully, painfully. It's better than losing skin and having nasty scars that you have to explain.