Good advice. That's pretty much what I was getting at. You need to do it enough that you feel comfortable with it and make it feel natural. If you've been running heel–>toe for many years it may take a little time to learn to contact on your midfoot rather than your heel.
I've seen quite a few people run fast like that too but it makes me wonder how much faster they could have run if they had learned to run landing on the midfoot earlier in their career. I wouldn't really recommend making such a major change on someone running as well as Sanya Richards and I don't think it's as detrimental in an event like the 400m (as compared to the short sprints) but I'm pretty sure that if they had learned to be comfortable landing on their midfoot earlier in their careers that they might have been able to run faster.