Recently I began competing on my local IEA (Interscholastic Equestrian Association) team and this year I started an ISHS (Inter-School Horse Show) competition team at my school. In both of those leagues you have to ride a horse that you have never ridden before, which can be very challenging.
Its not an easy task to be randomly assigned a horse you have never even seen, and then be expected to walk right into the ring and compete. Over the past year, my coaches have shared very useful information with me about how to successfully compete on a new horse.
1) Always watch the barn school their horses at the beginning of the show. It is important to see how the horse moves, figure out how he/she is about his/her leads, and figure out if you need a crop or spurs.
2) Watch people before you ride your horse. Even if the horse seems "easy", pay attention to what the rider is doing. Look at how the horse approaches the jumps, how he/she is in the corners, and what you need to do to have a successful trip.
3) Look at everything the rider does wrong and figure out what you can do to avoid their mistakes.
4) Ride conservatively until you figure your horse out. Just because your horse at home is ok with you asking for long spots, doesn't mean that your new horse will do well with them. Just pay attention to everything your horse tells you.
5) Adjust your riding. If the horse needs quiet hands, keep your hands still. If the horse needs a light seat, get into a little half seat. Learn how to change your riding to benefit your horse.
A great way to prepare for these competitions is to take lots of lessons and practice getting on new horses. It is easy to ride your horse everyday, but especially for these competitions, it is a good idea to try all different types of horses (ponies, horses, thoroughbreds, shetlands, slow horses, fast horses, etc.) so that you are prepared for any horse you are put on.