In order to get bigger muscles, you have to reduce the breakdown ("catabolism") of the muscle you already have, and increase the building ("anabolism") of new muscle.

Muscle anabolism (growth) is stimulated by resistance training (lifting weights) and by some body hormones (e.g., testosterone, growth hormone, insulin-like growth factor-1).

The best way to reduce the catabolism (breakdown) of your hard-earned muscle is to eat a lot of protein and complex carbohydrates, and not to over-train.

Generally speaking, you lose your muscle protein between meals. You recoup that loss after eating a protein-containing meal. A protein-containing meal doesn't increase too much the protein production, but it reduces the breakdown of muscle protein.

The oft-quoted Boirie study showed that your body needs a protein source where the amino acids are both slowly trickled into your blood and one where proteins enter your bloodstream very quickly following its consumption.

The faster a protein is digested and absorbed into your body, the more likely it is to be wasted. That is, more amino acids will tend to be "burned", or oxidized, for fuel. Thus it may not be advisable to consume a protein supplement that is all whey unless it is combined with other foods to slow digestion / absorption.

Most experts believe that natural bodybuilders (those who do not use anabolic steroids) need to consume 1 gram of protein per pound of bodyweight (2.2 grams/kg). Some research shows that 0.6 gram of protein per pound of bodyweight (1.3 grams/kg) is enough even for experienced natural bodybuilders, when it is consumed correctly.