That's a really good question, and there's not an easy answer to that question. One thing is, when you focus on training, to make sure that they understand that it's not a personal issue. They need to understand that it's a social issue or a workplace issue. If you can make them think of it in ethical terms, that someone is being harmed or hurt, then that's even better.
How do you make them do it? Just training won't necessarily make someone do that; it just makes them aware of it. I think you need to have a policy in place. I think I read that at the Department of National Defence they actually say that observers are responsible for reporting or getting involved in something that they witness. I think taking that step beyond the training would also encourage people.