• Intensity.  The higher intensity stimulates the body’s musculature to grow.  Intensity is increased by (a) progressively increasing the amount of weight you use; (b) progressively decreasing the amount of time it requires to perform a certain amount of work; or (c) by carrying each set to a point of total failure.
• Duration.  As intensity increases, duration must decrease.  Since we know that intensity stimulates the body’s musculature to grow, the less time you spend in the gym the better.  This is only true if you increase the intensity, thereby decreasing the duration.
• Frequency.  The number of times per week you workout is going to be important.  It is tough work hitting your body with extreme intensity.  Your body will need rest to recuperate from the pounding it takes.
• Adaptation.  Your body will adapt to the load being placed upon it.  Therefore, you must do everything you can to avoid that adaptation.  Change up the exercises, increase the weight, modify the frequency…it all helps to deal with adaptation.

When you apply the above principles you see bigger gains.  The more strict you are with these things the better your results tend to be.