Omg...I really want to try reining too!!! But, I don't have a horse, and I need a trainer that's kinda close to me. But, yeah...I really want to try reining-it looks so fun, and it's amazing to watch. Especially Stacey Westfall's bareback-bridleless ride!!! So, unfortunately I can't help you with what to work on before you start...as I don't even rein, but I would say do lots of transitions, circles etc. And, let me know how it goes, 'cause I really want to do it too!
Practice "cruise control" so that they won't break gait... that's important.... also make sure that you guys can do the fast and slow lope w/o a whole lot of spurring or pushing... it should look natural... practice spins and rollbacks in slow motion and then speed it up... flying lead changes are good to practice too, and stops... practice doing it by just sitting down and putting your legs forward and not relying so much on the reins