I've tried several things and sometimes combos of them. I sometimes use tape to create a little extra cleavage or fullness. To do this you stick the end of about a 20 inch piece of tape under your arm pit, then under one breast, then up the far side of the other. Then do the same from the other side to squeeze the chest flesh together. It works ok but movement can be restricted, the tape can be unsightly in certain clothes and the tape will slowly lose it’s grip.
I've also tried commercial chest enhancing bra-like straps that are tight and fit below the breast level and can push the flesh up but I just dont have enough flesh to push up.
Tips & Tricks
So mostly I use a combo of home-made breast forms and shading. The breast forms I made out of playdough, wrapped in a plastic bag to keep them pliable, then wrapped in an old nylon leg for the color. Then I squish it into whatever form would complement the particular bra. I use brownish blush and eye shadow to make an arc around the breast. The best way to do this is to place the heel of your palm on your nipple and rotate your hand up and out like a windshield wiper. Right hand does the right side and left hand does the left side. (not at the same time unless you are really talented). The shading should be lighter and more diffuse towards the top and darker and narrower as it approaches the cleavage area so that it eventually becomes almost a line at the bottom. I also use white eye shadow to highlight the part of the breast that I want to stick out.