Count, count, count the strides all the way up to the fence. It will keep you in rhythm with the horse the whole way there. It helps you figure out when the horse gets to the base and is about to take off. If you have an incredibly lazy horse (*herm*, like mine) that rather enjoys taking long spots just to throw you off guard, counting helps you tremendously.
Also, never ever ever look down or stare at the jump WHILE actually going over the fence. While riding, never look where you don't want to go. Horses pick up on this. Keep your eyes up at where you want to go! Good luck ;)
"A Bad Day Riding With Dad beats a Good Day Shopping With Mom."