Options for Correcting Sagging Buttocks?

Butt Lift Options for best results
There are many option to have a more shapely butt. They include a Brazilian Butt Lift if there is adequate fat available for fat transfer, buttock implants and a butt lift. Fat transferred to the Buttocks is generally called a Brazilian Butt Lift. Some surgeons, including myself harvest fat in particular from the lower back to give it an S shaped contour that defines and sculpts the top of the buttocks giving it the illusion that it is bigger and definitely more shapely before the fat transfer is actually done. Similarly, sometimes fat is taken from the upper back of the thighs to also define the edge of the buttocks while taking care of what many women call the "banana rolls" as well as the "diamond" shape space that is attractive at the highest point of the inside of the thighs when viewed behind. Also the amount of fat that takes is highly technique dependent and experience dependent.
Implants are generally reserved for those that want more fullness but have inadequate fat available for transfer. It is more involved and has a longer recovery. Lastly Butt Lifts generally refer to using "flaps" of redundant soft tissue that are tucked inside the but while tightening up the skin envelope. This requires the longest time for recovery and results in the largest scars which visible.
In other words there are many subtleties that make all the difference between a good result and a great result.  My suggestion is to go to a board certified plastic surgeon with significant experience in these techniques. His exam will determine which course if right for you.
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If you feel like the skin of your buttocks is sagging.  There are two potential corrections.  You can either remove the loose skin through a traditional buttocks lift or you can fill the loose skin with volume using either your own fat (brazilian butt lift) or an implant.  Good luck!

Your best options for a butt lift are either fat grafts or skin removal.  Skin removal is required if you have severe skin laxity or lack of fat in other areas to donate for fat grafting.