Hmmm you could do what RedneckCutie said witht he see-sawing at the mouth but for me I think that can be a bit dangerous at time. After a while your horse could become very sensitive to the mouth and start puttin his head up so you can't see-saw that's y I don't really see-saw at horses mouths...i prefer the pressure and relese...its simple and will NOT affect your horses mouth.
All you have to do is apply the slightest amount of pressur to one side of the rein and keep applyin presure till your horse drops the head even if its the slightest bit and the min he does, relese the pressure straigh away.
Once he understands the pressure and relese your horse will be great at bringin his head down.
But to me I really don't like see-sawing I think its a bit dangerouse that's just me though not meaning anything to you RedneckCutie please don't take this mean at all, this is just goin by my experiance.