It is essential that they really believe in it. When they get into a real race situation, they need to feel like they've been there before and were successful. Another part of good visualization that is often overlooked is that all the senses should be recruited. Many people just see mind pictures and they see them from an outside perspective, as though they were watching someone else. That obviously won't duplicate their race experience.
Good visualization is a skill that needs to be learned and practiced.
These pictures are self-fulfilling prophesies. Trying to ban the pictures is usually impossible. It's much better to replace them with a very brief mind picture of a successful outcome. Obviously, they shouldn't be riding around with their mind out of the race. However, a momentary picture can set them up for the move they need to make.