Apparently butt doubles are the thing to get! Keira Knightley has one for her lap-dancing scene in the film "Domino." Not out of modesty (the gal flashed her bosom in "The Hole" at age 15), but because she didn't consider herself ample enough:
"No, It's definitely a body double, I don't have that body. What a bum! I wish I had that bum! No, it was fantastic."