Working Out to Build Mass
If your goal is to put on as much mass as your genetics will allow, consider the following. First off, in order to gain muscle you may also need to gain some fat. This may sound discouraging but there are ways to keep the fat weight to a minimum. Get yourself in tune to eating about 500 calories more each day, and be sure to spread meals out to 5 or 6 daily. Be careful though, most athletes try to get these extra calories through fat sources. It's best to get these calories predominantly from lean sources of protein (e.g. chicken, whey protein, etc.), with some complex carbohydrate and essential fats thrown in the mix. Make sure the extra calories are coming from the recommended percentages of fats, carbohydrates, and proteins. An increase in body weight of roughly 0.5 to 1.0lbs. per week is a reasonable goal. Anymore weight gain than that and you're risking the dreaded spare tire.
For maximal gains in muscle mass, keep your reps between 5 and 12 and switch reps schemes every 3 to 4 weeks. This will keep the muscle guessing on what you're doing to it and keep you from overtraining. Now, here's the part that is guaranteed to get some of you hopping mad. DON'T TRAIN HEAVY ALL THE TIME. In other words, if you're training with reps of 10, don't train till failure every time you lift. And I don't care if you're natural or not. Your body will increase faster if you switch training intensity between light, medium, or heavy. Trust me, it works. You'll feel better, stay hungry for the gym, and you'll keep injuries to a minimum.
If your main goal is mass, mass, mass, then I would keep aerobic training to a minimum. In fact, you may just want to keep aerobic training out of your training completely. I wouldn't do anymore than 20 minutes, 2 to 3 times per week.
Here's an example of what a training program might look like. Now remember, this is not the magic program that you stick with for the rest of your life. In fact, I wouldn't stay on it any longer than 4 weeks. It's just an example. You need to switch things up every couple of weeks.