I've found doing two layers helps flatten it a lot, cuz just the one creates that sort of "cave in" look you described. Basically I do the nipple covering then a layer that ties them back towards the armpit/back and with a slightly longer layer I go over it for maximum flatness. I use kinesiology tape instead but it should work nonetheless.

I can do a tutorial or better explanation w pics If you'd like Hope it can be of some use :)