Address both nutrition and exercise
It's important to address both your nutrition and exercise routine to put on weight effectively. If you're training at least twice a week, but not eating sufficient calories or protein, you simply won't gain weight.

Balance calories
People who want to lose weight need to create a calorie deficit, and similarly, to gain weight you need to take on board more calories than you need to maintain your current weight. These calories should come from small, regular meals to encourage good digestion, support your body's muscle and reduce the amount of fat you store.

So how many extra calories do you need? If you increase your calorie intake by 500 per day - above the amount needed to maintain your current weight - you should put on 1lb (pound) or 0.5kg (Kilograms) per week. Effective resistance training in the gym should make sure this gain is not from fat.

Regularly monitor your weight
When you start on your weight gain programme, monitor your weight at the beginning and end of the week. If you've not gained a pound then add a further 500 calories to your diet for another week, then monitor your weight again. This way you can identify the point at which you put on weight without gaining unwanted fat.

Eat protein
How to put on weight effectively is not just about extra calories - the importance of protein can't be over-stressed. If your diet lacks quality protein you'll lose muscle tissue, lack energy and recover more slowly after exercise. Your immune system will also weaken.

Aim for a serving of protein in each of your five or six daily meals. Or for a more accurate calculation, multiply your body weight in kilograms by two to get your daily protein intake in grams, then divide this between your meals.

For example, if you weigh 55kg you need a daily protein intake of 110g, which means 18 grams of protein at each of your six meals.

Check vitamin and mineral intake
If you're training hard, it's also important to make sure you get all the vitamins and minerals your body needs, so you may want to consider taking a multivitamin.