Hey, this might not be a popular response but here it is.
I recomend following all the advice in the previous responses. Regular work outs -squats, machines, step up excercised too, a good diet, plenty of sleep, and just always be building up to more. Also, walking, hiking, and turing the treadmill incline will help develope your lower body.
Now with that said, I follow my advice, but I don't have a rock hard ass.
In fact, much to my shagrin, I have been told more than once that it is "feminine" or something like that! I'm thinking this is because it is kind of soft, and unlike the rest of me, not hairy. On the other hand, I'm going to bring up two good friends of mine. One does have a rock hard ass, and the other has calves the size of melons. Neither of these guys do any lower body work outs at all, other than walking around, jogging, ect.
Don't get obsessed about it. We all have what we start out with, and it's up to us to make the best of that, and be best we can be. Does that make sense? So instead of comparing yourself to somebody else, or focusing on for example, a hard ass, or the six pack, that so many guys obsess over, say, I want to increase my sqaut this month too....x. With process, rather than result oriented goals, your sure to improve, and see "results" in that, no matter what your starting point is, or what your eventual potential is (Consider a small framed guy is never going to be a heavy weight, but that doesn't need to stop him fro acheiving personal excellence)
Does that help at all?
Oh, also if your gym has cable machines, look for one that uses your foot. You put one in, and stand on the other. Then you you steadily pull your leg back behind you and and return to the starting position. It's a tough excercise. Good luck.
Matt | 
