I agree entirely with Siena Shirres. I own a very hot Thoroughbred mare who tends to prance around the arena rather than walk. In order to get her to calm down I murmur praise when she does good, and work her in circles round and round. Some days you might just have to focus on relaxation and making sure the horse trusts you. It's been almost a year since I got my mare and she's excelled wonderfully! Remember you can't be tense or the horse will be too :)
Try just talking to your horse and calming it with your voice. You also need to be relaxed yourself, sometimes I sing while I ride to calm myself, it is impossible to be tense while singing your favourite song at the top of your lungs!
Lots of circles and patience while riding and make the environment around you quiet to cut down on spooking.