As everyone else has said --> time and MILES.
And what ecasey was referring to above can be thought of as "wet saddle blankets". When a horse puts in a long, hard, day, with lots of cantering and lots of miles, and this is done on a somewhat regular basis, the horse will learn not to be too fiesty because they know they have a long, hard day ahead of them and will need their energy.
Long days and lots of miles are a great way to teach an energetic horse to calm down .... naturally, and by their own decision.
I used to have a horse named Scooter. Wonderfully trained horse, and he would do anything you ask, but I couldn't even walk on a loose rein when I got him. He'd been round penned to death, bascially. It took all summer to achieve a somewhat calm and relaxed horse, but I rode him tons of miles every single day all summer. He will never be a calm horse (not his demeanor as he was very high strung) but I helped to show him that it was *okay* to relax, and he didn't have to worry about me all. The. Time.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.