Halter or Showmanship --> practice, practice, practice. If you want your horse to set up quickly in world-class style, you need to spend a minimum of 30 minutes per day practicing. And you need to handle your horse like you are doing showmanship/halter ALL OF THE TIME. If you slack at all, your horse picks up those bad habits.
What do you mean he moves his back end while riding? He's not traveling straight? If that is the case, you need to use your legs to correct him and straighten him out.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.