As Kristy said, experience is key. Dedication is also very important. The man she mentioned had a passion for horses and to constantly grow and improve. Clients want to know your in it for the long haul, and have years of varied experience behind you. I can train Walkers, Quarter Horses, Thoroughbreds, Mustangs, PMU babies, Paints, etc. Each one breaks the same, but ive done experience with ALL of them. Ive seen kids that claim to be accomplished trainers, and have literally.. LITERALLY.. only ever trained two horses, and only handled about 6 horses, all of which were already accomplished leads. Another benefit to putting your hands on as many horses as possible, is EVERY horse is different. Each has their own fun set of surprises in store for you.. the bucks, bites, spooks, kicks, etc.. thats all experience. If you have only handled a few, or a lot of the same (like push button show horses) you wont be ready for the "surprise" some horses will throw you.
As for ads and whatnot.. try something different that would catch someone's eye. And when trying to keep clientèle, don't get a "God-Mode" complex. Ive seen many trainers that inadvertantly put their clients down by saying things like "If I had that horse, I would have NEVER let that happen" (referring to someone letting a horse get a certain vice) Having positive references helps as well.
I hope that makes sense o_o i sort of trailed off in that first paragraph. Blah... i slept in too late tonight ;;