When horses become upset or nervous they raise their heads.  You can teach your horse how to lower her head by creating a de-stress button on the ground and in the saddle.

First you will want to teach your horse how to give to pressure with a head drop exercise.  Stand on one side of your horse between her head and shoulders. Place one hand over the halter at the poll, and your other hand over the halter at the nose band. Apply steady downward pressure until your horse lowers her head, even if it's only half an inch. Your horse will learn the pressure goes away when she gives into the pressure by lowering her head. If she pushes back into your hands, continue to apply steady downward pressure until she relaxes the slightest bit. If your horse is really tall, try to keep your hands on your horse for as long as possible or stay with your hands raised towards her head until she relaxes and lowers her head the slightest bit. Repeat a couple of times, then do the same exercise on the other side of your horse.  Once your horse is really good with this exercise, to the point where you put your hands over the nose and poll and your horse lowers her head automatically (sometimes without even touching your horse), then you can advance the exercise to applying downward pressure from the lead rope.  Apply steady pressure until your horse lowers her head.  Repeat several times on each side of your horse.  This advanced exercise will need to be automatic as well before you can move on to the next stage.

Second  you will want to teach your horse the de-stress button so you can use it when you're in the saddle.  Pick a point on your horse's neck between the poll and withers where you will be able to easily reach from the saddle without leaning forward too much.  This is going to be your de-stress button.  Start on the ground and apply steady downward pressure on the neck over the mane, same as you did for the two head drop exercises above.  If your horse gets confused, then use one hand to apply the downward pressure on the neck and use your other hand to apply downward pressure from the lead rope.  Release both hands the instant your horse gives to the pressure and lowers her head.  You are helping your horse understand what you want by using a technique she already knows to get the same result with pressure being applied elsewhere on her body.  Repeat until she understands to give to the pressure from just touching her neck.  Repeat on both sides of your horse. 

Once your horse understands the de-stress button from the ground.  Tack and mount up and do the exercise from the saddle.  Practice, practice, practice at home in a safe area.  Now that your horse is solid with your de-stress button, you can use it out on the trail, at a show, working over obstacles, investigating new and scary objects to help calm and relax your horse without having to get out of the saddle.