The best way to increase lean muscle mass is to lift hard, infrequently, and get plenty of rest.  The body is an amazingly adaptable machine - the more stress that is placed upon it the larger and stronger it becomes.  But what most trainees fail to realize is that as the body gets stronger and able to exert more force, it's recovery ability remains the same.  This fact has profound ramifications on how a trainee progresses from intermediary to advanced - it advocates the exact opposite of what so-called "experts" tout as the correct way to train. 

So what does this mean if you're training hard and working out intensely - simply put, the stronger and more advanced you become the more time you will need to heal and grow.  It makes perfect sense if you think about it logically.  As muscle grows stronger it increases its ability to inflict damage on itself.  This ever increasing damage greatly increases the amount of time it takes for a muscle to recuperate.  For example, a beginner who can't possibly inflict a large amount of damage to his muscles may be able to fully recuperate within 48 hours.  He simply isn't lifting enough weight to matter.  Now, imagine an experienced lift whose lifting rather large amounts of weight - the recovery ability for both remains the same.  Which one is going to need more time to heal?  Obviously the more advanced trainee who damaging his muscles at a much greater rate is the one requiring more time - he may need up to a week or more to fully recover. 

The biggest thing is to remember is the fact that as you get stronger and gain size your recovery ability does NOT follow suit.  It's ability to do so is strictly limited and must be accounted for.  If you do not heed this basic law you are doomed to overtraining and will never realize your body's full potential.  Rest is above all else in terms of importance when it comes to proper training - it cannot be ignored. 

If you've been following a workout program that has you hitting a muscle group more than twice a week you're most likely overtraining.  I can also guarantee that you haven't seen progress either - you're just not giving your body enough time to recover grow.  It's a head game, plain and simple.  A lot of trainees think their going to "wither away" if they take more than a few days off.  This is totally false and absolutely baseless.  In fact, most individuals who take a week off or so come back stronger than ever before.  And the reason for this is because they gave themselves adequate time to rest, recover, and grow.  Their bodies finally had the opportunity to adapt and grow.