A great idea is to get a relaxing picture in your mind. Take a visualized ride. Imagine you are riding through a beautiful field and your horse is relaxed and imagine just how it would feel in every single way - how he would turn, stop, etc...then imagine you turn down the street for the parade, meet with all your friends riding alongside you, and imagine how calm and alert and obedient your horse is. Relax the muscles in your body and let out a deep breath as you do this...imagine every detail, the streets, everything.
Many athletes do this - they imagine their best ski run down an hill, for example. Some of them are so good at this, they can 'imagine' their way through a downhill ski run so well, 'doing' each gate on a certain hill, doing each turn, that the coach can time them doing an imaginary run, and it will be within hundredths of a second of the real time on that hill. They picture themselves having a faster run many times, and then they do it!