We hang water buckets slightly above tail height. Got really darn tired of horses back up to their buckets and pooping in them!
Another option you have is hang the buckets from baling twine (use two strands) around the center post in the stall and then you can adjust it up and down until you find the perfect height for each horse.
If your horse is the nervous nellie, gotta be looking out the front of their stall at all times, hang the water bucket on the very front of the stall. Found those horses will drink 30-50% more water than if it's hanging on the side of their stall.