The first thing to note that if the body is "hungry," it isn't going to build muscle. That means one should always have some sort of food in them- fasting is usually a poor idea. In particular, the body needs protein to help build the muscles being worked out. Steak, peanut butter, and special shakes or candy bars are usually good sources of protein.
Drugs are sometimes used to help increase muscle mass as well, but such drugs are usually under heavy regulation practices. Steroids, for example, can only be obtained in some nations with a doctor's prescription. This is because many of such drugs can be very dangerous and impose serious health risks. Some over the counter drugs can be obtained, such as Creatine, but these too may have side effects.
Hydration, like keeping nourished via food and snacks, is important in building muscle mass the healthy and safe way. One should actually drink small amounts of water all throughout the workout to prevent dehydration, since one actually feels the thirst for water they are already hydrated. Don't over hydrate your body, however, as this can have adverse effects on one's health.
The actual workout can now begin after one's body is properly nourished. A workout should be strenuous enough to tire the body, but not so much that it pulls muscles and ligaments. A trainer can help a consumer through the process of building muscle mass the most effective way, as they can take different factors about a person and make a customized routine for working out.
Be very careful not to overwork the body. There have been many documented cases where muscle will actually start to deteriorate from exercising it too much. Other times, muscle may expand too quickly, usually with the use of steroids, and seriously rupture muscle tissue. If this occurs, regenerative therapy will have to be ensued, and progress will be lost.