I once had a student that wasn't very sure of her capabilities. I was just plain sick of hearing her say, "I can't do that." I just told her "I couldn't do it either when I started. The fact that I'm doing it now tells you that it can be done though, right?"
Once she conceded the possibilty that aikido could be learned, there was no reason for her to doubt that she could learn it too. From that night on, I made her knee-walk once around the room every time she used the phrase, "I can't." Her shiko waza improved considerably.
Sometimes I wanna just grab negative people and shake them, screaming "YES YOU FREAKIN' CAN!!"
Thanks for the karate story, Saturn.