depends on what kind of horse and what you do with it! i agree, definitely post them online. Online classifieds are really popular these days. But the best luck I have is getting horses out where people can see them and just getting the word out they're for sale. So for my horses I take them to every show I can, even if I just school that day and tell the other trainers that he's for sale. (calling trainers and stables and just letting them know that your horse is for sale can be a GREAT way as well). Or if you're an avid trail rider find a trail riding group or organization and take your horse trail riding with those people. Especially if your horse is well behaved, people take notice and will remember your horse when someone is on the market for a new one.