Hi Molly, developing cadence in your horse's paces has to be done carefully, so as to avoid tension creeping in and running the overall picture and paces. You also need to be worry of pushing your horse out of his natural rythm and confusing energy with speed!
I think it all depends on your level and what you're looking for, but I found sites where he talked about the cadence, explaining it and giving ideas for exercises. The first is article for western training, but the basic exercises are the same for all riders :)
The other site comes from the magazine Dressage, it's Kathleen Raine who gives advice.
I hope this helps :)