As other's have said, use your hand on top of the head--IF that is an option for you. If you're really short it can be pretty hard! I was taught at first to just put your flat, open hand on the poll and say "Head? Head?" until it finally drop its head. So I did that for awhile and it worked, but my horse was sluggish about it. She wasn't in any real hurry; the scenery looked pretty nice from way up there, she thought.
Then I saw Clinton Anderson teach a horse to lower its head, but he did it just a little differently. He first just put his palm on the horse's poll, then lightly pinched the poll, then went to the next degree of discomfort, then went to the hardest pressure (if he needed to), and stayed there until the horse lowered its head. He immediately released the pressure after the horse gave in. I found this to work very quickly, and now I barely have to press her poll while saying, "Head", and she immediately lowers it.
**I must not forget to thank the difficult horses, who made my life miserable, but who were better teachers than the well-behaved school horses who raised no problems.**