I love running downhill. I think the downhill is very much mental. It's about controlling your fear of falling and being familiar with the exhaustion forced upon your quads.
Whatever hills you have, and however long they are (doesn't matter), just go bombing down them as hard as you can...over and over and over. If your quads are crying, you're doing something right. Someone mentioned stairs. That's great, too. Run down as many flights of stairs as you can as fast as possible (hotels are good). You have to hammer it. Again, conquer the fear and allow your muscles to experience the destruction necessary to turn them into decent-dominating beasts.
Another option, albeit much lamer, is to train with weights in the gym. Do lots of half squats and step downs with body weight + a little extra (maybe 20-45 lbs). Do, oh I don't know, maybe 10-20 reps for 3-4 sets or some such thing.
The physical training is essential, but I think the most important thing is to not be afraid of falling and to attack with such ferocity and velocity that you pass scores of people flying down as they're thinking, "What's wrong with him!?" Yes, there's something wrong with you. You're not afraid of running down a hill. You should be afraid. You should be very afraid. But, this extreme running business isn't for the timid now is it?