Lift heavy. Make sure you have full control of the weight so your spotter isn't doing most of the work for you. The rep range for adding muscle mass is from 8-12 reps per set. Make sure you feel like you have to give everything you have got on your last rep. If you feel you are not giving it all the strength and energy you have left within you, then you are not lifting a heavy enough weight, so try increasing it. Complete fatigue and exhaustion is what you must aim for at the end of each set. Take a 60-90 seconds in between each set and start your reps again when your muscles have had chance to rest.
Focus on your movement, ensure you are lifting weight correctly in a slow and controlled way. Lifting very heavy weights using a poor technique will only lead to injury, setting you back without being able to workout for a period of time.
Eat smaller meals throughout the day - Try to include around 30g of protein and 30g of carbohydrates in each meal. This will keep your energy and protein levels up, preventing your body from going into a catabolic state. Try to include a high carb meal about 1 hour before a workout, as carbs=energy, so you should get the most out of your workout this way.
Keep the calories coming in - An increase in calories is essential when building muscle, if your lifting heavy weights in the gym to work your hardest, but not eating much, the chances are that you won't see any improvements. This is because the body requires nutrients to grow, and if you're not giving your body these nutrients it needs, it simply won't grow!