I too recommend Pilates and a ball. It has done amazing things for flexibility (i.e., staying in the drops for a long time) and lower back strength. I supplement that by lifting in the winter and light arms in the summer.

Pilates is hard to learn by yourself, so I recommend a class. Once you know the routine, you can do either half of it after a ride to stretch or the whole thing in 45 mins to an hour on an "off day" and feel better and stronger for it.

There is also a thing called Bosu, but it was WAY too hard for me!