Uphill will do it. I have my treadmill set on an incline for the same effect.
More butt news - Oxygen Magazine has a Special Collector's Issue called "Glutes - Training for Women" that I bought at Barnes and Noble. Articles include:
No weights required - from Flat to Fab in 4 steps.
Cover Girl's advice for rock hard buns..
Build your Best Butt Ever - 10 Surefire Moves to sculpt your rear end
plus more... and a glutes workout poster.
Yep, I'm on a butt kick right now!