Tips for gaining weight in a healthier manner

1. Eat protein to gain muscle. If your goal is to put on muscle mass, then you need to be eating protein-rich foods on a regular basis. It's especially good to eat protein after a workout. Lean meats (such as chicken, lean pork, and fish), eggs, beans, and lentils are all great sources of protein, as well as yogurt and nuts, chocolate milk is a good way to get in your protein, although be mindful that most chocolate milks contain a lot of sugar, which isn't good for you in large quantities.
Also try to eat protein right before going to bed. Drink some milk or eat some yogurt to fuel you through the night. This may also help you recover if you work out frequently.
Stir in protein powder to your curd, oatmeal, and other foods to add a protein and calorie boost.
2. Add calorie-dense foods to your meals. Try adding full-fat cheese to the top of your meals. Stir some peanut butter and honey into your oatmeal. These foods are high in calories and can help you increase your overall caloric intake.
Another calorie-dense food is dried fruit, such as dried apricots, figs, or raisins.
Eat complex carbohydrates, such as brown rice, bulgur, barley, whole grains, and quinoa. Avoid simple carbs like white flour, sugar and white rice.
3.
Eat solid meals. Every day, make sure you get in at least three meals. Introduce larger portions at your meals to help increase your calories. You may have a tendency to skip breakfast, only eating 2 main meals, but making sure you actually eat three meals can help you gain weight.
If you can't eat huge meals because it upsets your stomach, eat smaller meals throughout the day. Don't skip meals.
4. Don't drink before eating. Drinking water or any beverage before eating can fill you up. You want to leave room for the calories you need to consume.
Instead of drinking water before you eat, try drinking a calorie-rich drink while eating, such as fruit juice or a smoothie.
5. Identify why you lost weight. If you are trying to gain weight back that you lost, you should first identify why you lost the weight. If you haven't seen a doctor, you should, as unexplained weight lost can point to any number of diseases, from an overactive thyroid gland to diabetes.
6. Treat the underlying disease. If a disease caused you to lose weight, treating that disease may help you gain the weight back. Talk to your doctor about an appropriate treatment for whatever plagues you, as well as what treatment would be effective to help you gain weight and what the most effective way is for you to gain weight.
Source: wiki
Besides these natural ways you can start taking homeopathic preparations like alfa alfa and carica papaya which which help you to increase your weight.
There are certain other remeies also which help in gaining weight, those depend on individual constitution for which you have to consult a homeopath.