The number one thing that the average elephant-loving human can do to instantly make the lives of captive elephants better is to stop invading their personal space to give them food. I will support this with three reasons.
Reason the first: an animal does not distinguish between a treat fed from the hand or placed in bucket. They just don’t. Adult animals do not hand feed each other. To put it loosely, one of the roles of matriarch of a herd is to keep the herd alive by finding food and water; if human carers are providers of the food, there is no distinction in the animals mind as to the mechanics of how that food is transported to their mouths. To them, the food is provided by the human carer and they are grateful for that. A captive/domestic animal will learn which humans carry treats and seem to lavish attention on that person, but they would be equally happy if the treats were deposited on the ground and they were left in peace to gobble them up.
Secondly, lets get into training. Positive reinforcement is a term bandied around a lot these days. But it is just a part of the type of learning known as Operant Conditioning, along with negative reinforcement, positive punishment and negative punishment. Operant conditioning teaches by using consequences to modify behaviours. I won’t get into the nitty gritty of it all here, but encourage you to study the practice more if you are curious about the science of how we learn. “Pavlovs Dogs” is a good starting point.