This is what we do, just a bit of one normal riding session.
Walk your horse in a circle making it smaller and smaller until your horse almost has to pivot on the hind end to finish the circle. You will want him to eventually do a proper crossover and roll back, then come straight out into a walk or jog . After a few of these you will see him drop his head because it is a more natural way for him to balance. Do it again and again at the walk, trot, and change transitions as you do it as well as direction.
A horse will naturally put his head down to do a small circle or 360 for balance and that is the key to getting your horses head set. The more you ask the horse to get under himself and engage his hind end the more he will want to put his head down to be balanced and light. Keep your horse calm when you do this, and be light with your hands, and consistant with the cues.
Eventually he will figure out that he has better balance with his head level and he will go that way naturally without too much interference with your hands. Of course you must ride with contact, but very light and you must release when he puts his head down to reward him, and to send him the message that head down is what you want. Timing will be everything when you ask with your hands for him to drop his poll, ....perfect give and take and it will happen as Tiny has said too much and you will make him behind the vertical. The key is to keep him working off behind.
Hope this makes some sense to you as it isnt always to explain how to do something no matter how many times you do it yourself or show someone else.