This complete guide will show you how to gain weight the right way—with a Skinny teenagers, underweight adults, and hardgainers of all stripes will increase muscle mass—plus, that kind of diet can get pretty expensive. . Get some sleep: As in any fitness regimen, sleeping is an essential part of the recovery process. Apr 14, - "Most lean men who can't gain muscle weight are simply eating and exercising the wrong way," says an expert. Read on for the right way.