Diet. If you want to lose fat off of anything, dieting is basically the only answer. Working out will make the muscles underneath more firm, which might help to make something look less droopy, but it won't get rid of the fat itself.
In terms of the weight-storage thing, time is the only answer. Fat goes wherever there are already existing fat cells to store it. So until HRT has had a LONG time to create more fat cells in gender-appropriate areas, it's just going to go right back where it came from every time you lose and gain weight.