To lose weight, replace two meals per day with a Slim Fast shake or bar. Make sure that the one meal you do eat contains plenty of fruits and vegetables to help you feel full. To maintain your weight loss, eat two healthy meals per day, replacing the third meal with Slim Fast. It doesn't matter which of the three meals are Slim Fast meals and which are regular meals. Slim Fast also makes snack bars, which you can eat three times per day. Dieters who choose the bars will discover that the biggest challenge is stopping after eating just one.