The common informal term for these are "cutlets" because of their similarity to raw chicken breasts. They are basically smaller and thinner versions of the silicon breast forms used as breast substitutes. Because of the massive sales of them as "enhancers" their prices are quite reasonable.
They won't take you from zero to double D (one their own, anyway), but they do beat nothing. I've used them for years and they work great. There are different types, but mine are nearly full form size and even have nipples. They add about 1 cup size so if you started out at AA you'll might fill out that A cup. In fact, some people cheat and pad them a bit from behind for a bit of extra oomph.
Awhile back on another forum I ran across some amazing photos of a crossdresser named Maryann Sung who created incredible cleavage by combining cutlet pads with a couple of other things. A Google Image Search of "maryannsung" might show you an example. Here's her technique verbatim:
Quote:maryannsung cleavage technique
Very easy. No HRT for sure. Here's the trick, no need for Nubras. Buy a good molded bra with liners, make an incision in the liner, insert a non-sticky gel pad, then cut up a 2" diameter foam squeeze ball into two halves, place the half balls into each pocket of the bra with the convex side facing your chest. On the surface of the liner, stick on each side a no-slip heel cushion you can get from Payless Shoes. The friction from the heel cushion will keep the pushed in breast tissues in place and the pressure from the half ball will create a crest on each side of the chest. This will make each side looks like a very sexy rounded cup. When you have them on both sides, it's perfect cleavage. Hope this helps.
I haven't tried this exact technique, but I did make a shopping list of the items she mentions here:
-Good molded bra with liners
-Non-sticky gel pads (pair)
-2"D "Sqeeze ball" (cut into 2 halves)
-No-slip heel cushions (pair)
-Maybe sew in some velcro to close liner flap