When teaching a horse to back up, or trying to perfect backing, you never want to get into a "tug-o-war" match. You ultimately want your horse softening at the poll and willingly moving his feet in reverse. You do not want their head up gaping their mouth open due to heavy hands trying to force them into backing, all this does is make their back hollow out which in turn makes them drag their feet and feel sticky. What I have found works best for me is to apply pressure to the reins towards the saddle, not in the air, and ask the horse to back up, if they do not back or their head comes up, alternate pressure from one rein to the other, what I call "see-sawing" the reins, be sure to apply the pressure gradually and not to jerk on your horses mouth. Next, apply pressure with your legs a little forward, and the last step is to shift your weight in the saddle from side to side as if you're wiggling in your seat, this encourages them to move their feet as well. Be sure to reward the horse by releasing pressure at the slightest try so they will be more willing to do it the next time, and soon enough, time and patience will pay off and you'll have your horse backing up like a pro!