I can give you advise for what i used to do.......
1. Buy a cleavage enhancer for about $39
www.suddenlyfem.com 2. the cheaper route i used
a. get old bra one size to small IE im a 46 B so i would use a 44 . Cut off the cups and leave band and strap
b. Take brown concealer or a darker shade closer to skin tone trace with shadow brush starting right below neck line down. on both sides of breats
c. Blend in with a good blush circular patterns outward towards middle.
This is what i used to do, the purpose of makeup is to enchance appearance and cover flaws i saw no cleavage as a flaw so found a way to fix it.
Marsha is the pro she might have a better idea.
ps. another one of my drag queen friends tips.