Varying your set and reps will help you bust your plateaus. Say, you have been working your chest and you are using four sets with declining repetitions. I.E. 15,12,8,5. Instead of that type of repetitions, you could go with heavier weights and lift 12,10,5, 3. Using heavier weights with lower repetitions should help you gain muscle mass.
Lift to failure on your last rep. In order to gain muscle mass you need to fatigue your muscle groups to failure. It’s always best to have someone lifting with you so they can help you complete your last set.
Train with a partner who has similar goals. When two people who are like minded begin sharing ideas and techniques, it will both help the reach their goals and bust through plateaus.
You can pretend that you are eating properly, or you can actually eat properly. A big problem with trying to bust through plateaus and gain muscle mass is skipping carbohydrates immediately after a session. You need to replenish the carbs you lost during your weightlifting session. But, the repletion process must begin immediate after your session, not two hours. The hour after your last session is the optimal time to put the lost nutrients back into the body. It will aid in muscle development and muscle healing.
Ditch the cardio. I know you like to get that ripped look that the opposite sex really digs. But, when you are busting plateaus and you want to gain muscle mass, you need to skip cardio. Cardio can be a deterrent when you are trying to gain muscle mass and bust plateaus. It’s especially true when you are trying to build legs.