Eat and eat often. While everyone should abide by this rule, it is especially true for those who are trying to gain weight. Eating often means eating five to six small meals a day and packing in those nutrients.
This doesn't mean eating junk food and sugars, it means lots of protein and carbs.
For those who want to gain weight without accumulating excess fat, the key is to increase weight through gaining muscle mass. For this, it is necessary to practice physical exercises several hours a day that cause a great effort and muscle wear, such as weight training and crossfit, besides having a protein rich diet, such as meat and eggs.
In addition, in some cases protein supplements may be required to increase the stimulation of hypertrophy and accelerate muscle recovery after physical activity.
