From what I understand the most important thing to gaining mass comes after the workout if you do the right exercises while working out. During your workout you should do exercises that have a wide range of motion, this creates tiny tears in your muscles, which is the key to gaining muscle mass. After you workout, if you intake 30-40 grams of protein (shakes work best because liquid goes right into the blood stream) it will "fill in" those tears you created with the wide range excercises and stretching, thus creating muscle mass. It's important that you don't take anymore than 30-40 grams of protein at any given time because your body can't handle anything more, and you'll just crap it all out and waste your money. On one final note, make sure you consume as many grams of protein in one day as you are pounds in weight (exp: 180 lb. person should take 180 grams of protein). Pretty much a combination of eating one meat meal per day, a few glasses of milk and 2 protein shakes a day should take care of that for you. FYI I'm not a professional at this, but I've done some pretty serious weight training in my life, and I've gotten great results using the stuff mentioned above.