My boyfriend, and I listen to him because he's a personal trainer, always tells me to do a balance of cardio (treadmill, stairmaster, elliptical) and strength (weight training). Your diet also helps out a lot. He always tells me it's 75% diet and 25% working out. On a typical day, I'd run 30 mins on the treadmill and then he strength trains me. I do squats and lunges w/dumbells, squats w/that bar thingy you pull down, some butt blaster machine - stuff like that. Feel free to PM me if you have more ??'s - I can always ask him for you too. HTH!
Running is awesome but can be tough on your joints. I run anyway though, because it is enjoyable for me (with my ipod.) I also like carmen elektra's fit to strip video- it has some butt exercises, and the butt machine at the gym. Squats are good too and I saw this thing in Seventeen magazine about toning your legs by doing these jumps (squatting then jumping as high as you can, legs straightened but soft landing)- there were other jumps too and I did these once and my legs felt DEAD the next day. Combined with cardio and diet like ShopChicago said, I think you'll definitely see a difference.
Kickboxing and weight training. There are two machines at the gym that are even finally clearing up the last of my cellulilte - one is an outer thigh press and one is an inner thigh press. Personally, I cardioed my way down to a size 0 four years ago (I have really small bone structure), but still had cellulite and lumpy thighs. My body looks way better now than it did then, and I'm a size bigger.