Simply adding certain foods to your diet can be very effective to gain weight. Eggs are one of the healthiest muscle-building foods on the planet. Milk has been used as a weight gainer or muscle builder for decades. It provides a good balance of proteins, carbs and fats. For quick on-the-go energy and weight gain help, grab a banana. They are high in potassium, carbohydrates and other important nutrients. Give yourself a quick calorie boost by eating a handful or two of dried fruits. Salmon and oily fish are excellent sources of protein and important healthy fats. Avocados are fairly calorie-dense and therefore a great food to help you gain weight.