Weight training puts a huge strain not only on your body but also your endocrine system. Adequate rest and recuperation after your workouts is essential. If you consistently overtrain (too much volume and training too frequently), a couple things happen:

* You don't give your muscles enough time to recuperate, repair, and grow between workouts. If your muscles have not repaired themselves, you will not be at maximum strength for your next workout. Rest is absolutely essential for muscle mass growth and increased strength.

* You run a pretty good risk of an injury.

Building your ideal physique is not an overnight deal. You need to pace yourself. You want to be able to keep working at this consistently, not burnout before you reach your goals.

I usually only train with weights 3 times per week. Any more than that and it's not enough time for my body to repair and build new muscle.

You don't grow while working out, you only grow when you're resting.

Below is a good example of a workout specifically for increased muscle mass. I recommend doing 4 heavy sets for 8-12 reps each.

Monday(legs, abs)
* Leg extension/Squats superset
* Seated Calve Raises
* Crunches (4 sets of 20)

Wednesday (chest, shoulder, triceps, abs)
* Incline dumbbell flyes/Flat bench press superset
* Side dumbbell raises/Shoulder press superset
* Tricep pushdowns
* Reverse incline leg raises (3 sets of 20)

Friday (back, biceps, abs)
* Dumbbell pullover/Wide grip pull-ups superset
* Barbell curls
* Crunches (4 sets of 20)

For some hard gainers 4 sets of each exercise may be too much volume.  If so, cut back to 2 sets of 8-12 reps.  The superset exercise combinations are pretty intense.  It may be best to start with only 2 sets of each exercise for the superset combos and build up to 4 sets over a period of a few weeks.

That's it.  It's not complicated or fancy...just effective.

All the best,

Doberman Dan