To me, I like my back-up to feel like my horse is folding up into a spring. I want the haunches under him, back rounded, moving at a reasonable pace for his training, head down, and in a straight line. A horse with impulsion will use himself better than a horse trying to get out of using his hind end (by swinging out, hollowing the back, bringing up the head, etc)
Helping make the back up better requires the horse to move forward well. I want my horse to spring forward when I ask him to go, and stop with his haunches underneath him.
I prefer to use a combination of rein and leg cue and eventually phase out the rein cue to make the back up a leg cue. Usually by jingling my spurs or tapping lightly with both feet. The rein is used at first to reinforce. Hold, release, hold, release, etc. with each step to encourage quicker movement.
I like to get a little momentum going, so I'll ask my horse to trot forward, halt, back three steps, and then trot forward again. Soon they start anticipating the forward movement and start sitting on their haunches to "launch" you forward.
And then you should have control of your horse's hindquarters. A "third" button, like if in front of the girth controls the shoulder for a pivot, at the girth is forward/sideways, and slightly behind controls the hip left/right/disengagement. A good exercise is to try and make a circle backing up, keeping it round and smooth. It's quite difficult!