Going to jump in here with some basic info. Somehow I missed this when it was originally posted so I'll rectify that now.
Honestly the simplest method is to stop wearing a bra, or at least as often as possible. There is a ton of conflicting information about this online but I have around 35 years experience with this so I can tell you what works.
There's a couple of reasons for wearing sports bras in particular, the first is to prevent discomfort and possible bruising which if you're reading this is probably not something to dissuade you.
The second is that the bouncing up and down of the breasts when exercising causes the coopers ligaments to stretch and even break. While the breasts contain a number of muscles the form and tone, it is the coopers ligaments that keep breast in shape.
As you age and breasts are subject to more and more bouncing, or just gravity unsupported the ligaments breakdown and the boobs head south.
A few other things to consider, the bigger they are faster they fall. Now if you want to redistribute where fat is stored one secret weapon I know of anecdotally is using Marvelon birth control pills. Yeah, you won't find any research on this but when it came along a lot of girls I knew called it Marvelous Marvelon due to the sudden increase in cup size.
Yes you can play around with binding and all kinds of other games and they are fun but unless you're doing them daily you won't get any worthwhile result.
Stick to the basics, work to break down the coopers ligaments and do what you can to boost fat deposition in the breast. Combined you can see results in 6 months, a couple of years and you might be tucking them into your waistband...
Good luck.