
Eat the right food
Increasing your consumption of junk or greasy food won't do you any good in your weight gain process. Junk food usually contains saturated or unhealthy trans-fat that is bad for you. Even though you need to increase your calorie intake, make sure the calories you consume are healthy.
Sources like Tuna, Salmon, Flax and Walnuts are excellent sources of healthy fats that include Omega-3 fatty acids. Lean meats, poultry and nuts are great sources of protein.
If you aren't used to eating 3 heavy meals, splitting up your meals in five or six smaller balanced portion would be a great idea too.
Exercise to gain weight
Exercise is very important for any healthy weight gain program. You can increase your muscle size by lifting weights. Try keeping away from aerobic exercises such as stationary cycling and running as they are very useful when required to lose weight. So don't do much of these as you may end up losing more weight.
Resistive training can be done wherever you got the right equipment. At home or at the gym. If done properly, with a large deal of patience, weight gain could be a reality for you.

