I think I am going to start clicker training my horse The Willing Equine and JETequitheory have inspired me. But you do have to do good research. And I am still one the research stage, before I start the actual training.
i haven't personally gotten to try it yet, but i follow a lot of people who do clicker train their horses and use positive reinforcement with a lot of success. here are some resources to get you started:
tumblr, the blogging site, has a surprisingly large community of people who clicker train their horses. go to tumblr.com and search "horse clicker training." the blogs clickerpunk, foxy-filly, themotherfuckingclickerkid (sorry for the language lol), and thequeerblackquestrian are particularly good.
not my personal horse, but I work as a trail guide and I taught the trail horses a few tricks last summer through clicker training. It broke up the monotony of their day and was fun. Plus. it makes a little girls day when the horse will give her a kiss before a pony ride. Just make sure they don't get too grabby with treats