I clicker trained my horse, and it REALLY helped when teaching him not only to give me his feet, but especially to hold his foot there. Once he got the idea, I started waiting longer and longer before I clicked and treated, but the goal is to always do so before he tries to pull it back. If I accidentally wait too long, I'll try to hang on to the foot and wait until he relaxes before giving it back. Now, I don't have to use the clicker (unless he gets REALLY bad about it, which hasn't happened in a long time), and have been cleaning his feet out daily while they're eating their beet pulp mash. He's very food motivated, so he's learned he can continue to eat if he's standing patiently. If he pulls his foot back or tries to move, he either naturally steps too far from his food to eat or I drive him away. When the farrier comes, I plan on letting him eat while he's worked on at least while we're still at this stage. Eventually, I'll wean him off of the food all together.
Honestly, the thing that has been the best for him as been finding something to keep him occupied rather than just standing there as well as working with him on a daily basis.