Eating five or six meals smaller meals per day when you are underweight can be more manageable compared with the standard three. This is because you typically feel full faster than a person at a healthy weight.Snacking on avocados, nuts, cheese, peanut butter or dried fruits between mealtimes can be a good way to gain weight.Strength training (building muscle) can help you gain weight. Exercise may also help you get your appetite back.Fluids before meals can affect your appetite. Try limiting drinking until 30 minutes after you’ve eaten.Strength training (building muscle) can help you gain weight. Exercise may also help you get your appetite back.