One of my horses gets very quick and excited too. Whenever she gets quick, I make her circle into progressively smaller circles, think "slow" with my body, and ask her to get into a frame. In the beginning I'd have to do many circles until she slowed down, but now I'm at the point where as soon as she realizes she's circling (like 1/4 of the way into the circle), she drops down several gears (part of which is due to the half halts to put her on the bit, which she understands better on a circle). Then I proceed on with whatever I'm doing. Repeat as needed.
I'm a big, big, fan of circles.