Just a word of advice, please don't just go buy a clicker and some treats and start clicking. Clicker training is all about timing and you really need more information about it or you'll perhaps accidentally encourage the wrong behavior. You also need to learn how to deal with a treat pushy horse if you decide to use treats as your reward. Some horses do get very mouthy at first when they learn food is involved. You also need to learn how to phase out treats. Please also learn from someone who knows what they are doing. I highly suggest you go to Amazon.com and purchase Alexandra's book "Clicker Training for Your Horse." It's worth it's weight in gold and will help you from making newbie mistakes we all make when we start. There is also a Yahoo clicker training list and we have some awesome experts on that as well as many newbies posting questions. If you're interested in that please message me privately for the link.
Also there are many clickers on the market. I use one for the horses with a farily loud click so we can hear it at distances but the good thing about clicker training is you don't have to have a clicker. You can use your very own voice. Also, you don't need treats necessarily. You can use anything that the horse perceives as a reward.
I'm just trying to say like any other method of training, there's a learning curve and there's some great teachers and resources out there to help and then once you are on your way, the sky is the limit. Clicker training builds awesome relationships with your horses..