It involves a lot of you using your seat and leg to really engage them! Draw reins are great to help with a headset, but can cause horses to actually disengage their hind end if not used correctly. When on the flat, I focus on a lot of extension and collection, figure eights, serpentines, etc. Anything to keep my mind and my horses working. A lot of these exercises will help with engaging their hind end as well.
I would use draw reins. They work well and the horse listens to you!