Watch this video about diets from Oprah's trainer on HowStuffWorks. Anyone who's ever been on a diet knows it can be frustrating - you lose a few pounds and feel good, then gain a few and feel bad - and most health experts agree that this rollercoaster effect is not healthy. Bob Greene, Oprah Winfrey's personal trainer, says dieting is about changing your life, not only with how the body works, but also how the mind works. See how diets work in this video from Medialink.