I have been trying to gain weight for years now and nothing has worked.
To gain weight, eat 500 calories more that you burn everyday. Choose calorie-dense foods like nuts, peanut butter, dried fruit, dense starches, lean meat, and oils. Eat large portions at mealtimes and graze throughout the day. Choose juice, milk and shakes more often than water or tea. Do weight-training exercises 2 to 3 times a week to build muscle instead of gaining mostly fat. Read about how to gain weight from the Nutrition Guide at About.com