Time is the key, as with everything else with horses. They learn by repetition, so you just have to keep on doing it until it becomes routine and not a big deal.
I would see if you can go with someone who has a horse that you know will ignore them.
In SA we trained them to get used to big game (inc. lions) but just taking them out in a group of experienced horses and let the new horse observe the others not caring at all.