I really hate the idea of focusing on a horses head. The head is just a pretty accessory for the body. The neck is used to balance and if the body is moving correctly, the head should come into place.
*sigh* However (and I will probably get yelled at for this, I am aware), I am currently employed at a barn that is pretty high up in the AQHA world. On their WP horses they teach them that by giving a squeeze with both calves the horse should lower their head. This way your hand stays still when you are riding one-handed in the show ring. I rather see this than toying with their mouth.
If she is put together in the way that she naturally wants to carry her neck level (which a lot of the horses bred for this are) you should be able to ask her to round and move from her hind end and her neck/head will fall into place on it's own which is far preferable.