|I just read Diane's technique for altering a pants pattern to remove that "clown butt" look on pants. Here's one that might help if you've nearly finished making a pair of pants, and find the butt sag remains (on the garment, can't do anything about the body... sigh.. wish it were that easy).
If there's sagging under the butt, pinch out the excess at the back of the thigh till the folds disappear and measure the amount you've pinched.
This amount can be removed from the back at the waistline. Lower the back waist by the pinched out amount. Of course, this'll probably create a wedgie, so you'll also have to scoop out the crotch in the back. Do a little bit at a time. It may not be the "perfect" fitting pants, there's only so much you can do once the pants is at this stage. But it should be presentable enough for everyday wear.
Or... well we could try and get our exercise and lift those cheeks again.... yeah... right.. hehe :)