Hey fastjessie, I ride an excitable lesson horse all the time. To quote my instructor "She thinks everything in life is so fun she needs to do it at top speed".
. I also hate half-halting constantly and pulling on her rather soft mouth, so when she gets dancey I slow down my posting and try to slow her down with my seat more than just with my hands. If she gets really bad I will circle her until she decides this isn't fun anymore and slows down. From consistently working with me she's working with me a lot better and trotting at a more consistent, slower speed that's a lot easier to work with. Just some advice and sorry if I sound obvious, I just want to help :). I agree with what everyone else said, too.
Practice makes perfect, and remember that everytime you ride a horse, wether you realize it or not you are training it!