there is really not much you can do at all. pure silicone oil (found at a hobby shop for R/c shocks) might help avoid further drying. once rubber is brittle its already started to rot away and there is no way to reverse that.
On my Kirby 510, which was in pretty nice shape when I got it, the trim had discolored and looked green and kinda dry feeling. Using the tip of how to clean dirty power cords, I used lacquer thinner which removed the discoloration and made it look new again (perhaps it just removed the top layer or something, but it looks great now.) You could then use a silicone product but in my case it did not need it.
