Here's what I did to teach my horse to stop. I would ride him along the fence at a walk/trot/canter and when we got close to the corner I'd pull back and say whoa. If he slowed down/stopped, then we'd turn the corner, but if he didn't, I'd keep him going straight like he was gonna run into the fence and he'd slow right down.
Great advice. Now can we get help for horses that do immediately stop at a whoa but never stand still and dance around? My biggest peeve is when they stop and then keep moving backwards. Especially bad in a cart without brakes.