Well, they don't really go away....
So, you can use armotization suppressors to shrivel the ducts and tissues.
Testosterone will cause them to deflate and droop, think "Moobs."
And lots of targeted exercise cna make the fat depsoits dissipate.
You can look into massage, reverse "chi masaage" supposedly takes them down in size (IE, reversing the massage direction - the point usually being, the chi massage is done for GROWTH.)
I've seen articles on dry-brushing to BUILD the breasts, by pushing fat towards the breasts; reversing that might work, but I'm no expert - I've SEEN articles, not even read them.
also look into androgenic herbs.
Or, just keep them. Gyno is common these days, wear a sports bra or a gyno compression vest (they cost about $80 and up - i'd say go with the bra, you can get about 8 decent ones for the price of ONE compression vest.) the bra will minimize projection and bounce, and if you put on some belly fat, you'll be a guy. (From an old Trans* site I recall, "If you have a belly, you're a guy. If you don't, you're just a girl with no hips.")
some girls even like guys with "responsive" chests. I have not MET them, but I'm told they exist... ;-)