I'm with DKDJ - ellipticals are great low-impact conditioning tools, but they're not usually used as a 'building' exercise. I know that hip thrusts, bird-dogs, and glute-ham raises are often recommended for the posterior chain (ie. lower back, bum and back of thighs). Also, up-hill sprinting and jumping (as high as you can, kicking your heels back to your bum!) are great ways to help get a 'better bum' AND spend 10 minutes making your heart race for free.
The gluteal muscle group (as far as I know) tend to respond to slightly higher reps and moderate resistance. I'd be happy to rustle up the references I've read on it (somewhere....) if you're curious about specific routines. 'Hope that helps!