What can I do to fix my saggy butt? Lost a lot of weight. (Photos)

 First, congratulations on the weight loss. However unfortunately once the skin is over-stretched it is unable to recoil back, even with healthy nutrition and exercise. Usually what I recommend in these cases is to "reinflate" the buttock first by placing a buttock implant restoration of volume and fullness. Then  6+ months later remove whatever residual skin remains with a "lower buttock tuck". Glad to be of help.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

No amount of exercise can improve the appearance of redundant or loose skin.  Hence, you may consider buttock lift in which the scar is made within the groin or skin fold for its optimal concealment.After complete healing (about six months), you may consider buttock augmentation via fat transfer (or more commonly known as Brazilian buttock lift, although it doesn't actually lift the butt) to further improve your backside's contour and projection.Your best bet is to consult with board-certified plastic surgeons (ABPS members) who regularly handle massive weight loss patients requiring body lift.  Compared to "average" patients, MWL patients require more extensive approach or additional technical details to achieve optimal results.Good luck.