A lot of people inject into the upper butt area, right where the tops of the pockets would be in your jeans. Some also like the thighs.
I'd definitely worry about the actual butt cheek, since if you're lean it's probably all muscle at the surface, instead of fat.
Another thing you can try is pinching your belly area, find a good fold of fat, and inject at an angle. This way you can be sure to get the fat and not muscle...
I'm not thin, so I don't have your problem... Once I get all this weight off, I hope to have your problem!
