There are different techniques used by surgeons when harvesting fat, and the ultimate goal is to keep the cells alive and to isolate the fat cells so that their opportunity for survival in the new location is at its peak.
However, while there are different techniques used, when it comes to location to harvest fat, anywhere that the fat is abundant is usually fine. Often patients will combine liposuction to remove unwanted fat, with fat grafting to use the fat elsewhere with success.
Interestingly, the fat will keep its same weight gain characteristics as the donor site, no matter where you put it as a graft. What this means is that if your body is prone to store fat in the abdomen and a great deal of this fat is used as a graft on your face, then the grafted area of your face can gain weight disproportionately with the rest of your face. However, if the grafted fat is going into the buttocks such as with a Brazilian Butt Lift, then the weight gain might actually compliment your figure even more.