The problem is that very few people actually practice bumping. But they should

Next time you're out riding with your friend, tell him you're going to come over on him. Bump elbows. Retreat. Repeat.
Eventually, you can get to the point where you're slamming into, leaning on, and pushing each other with force.

If you're afraid to get hurt, do it with beater bikes on a grassy area.

So when it happens in a race, you won't freak out.