Though p90x isn't a bodybuilding workout, it does tap into some bodybuilding philosophy. First, let me describe a simplified version of how bodybuilders approach weight lifting. Bodybuilders train by isolating specific muscle groups and blasting them for one day, and then giving ample rest on that muscle group before working it again. They also pick heavy enough weights to only be able to perform 6-10 reps per exercise, to create microtears on the fibrous muscle tissue, which then repairs, growing stronger and bigger than before during the rest period through a process called hypertrophy. A bodybuilder's diet starts with a high calorie, high protein diet to ensure the addition of mass, and then before the competition involves a ketosis diet - which is a healthy low carb, high protein diet that causes you to use ketones, or fat molecules, as fuel during a workout, as to burn the layers of fat over the muscle.
P90x does blast specific muscle groups in series with adequate rest (chest and back, then shoulders and arms; chest/shoulders/tri's, then back and biceps), but is more of a circuit weight training routine than a typical bodybuilding routine. P90x is fast paced and moves quickly from one move to another, and doesn't repeat exercises more than once. Additionally, p90x contains many compound moves which don't isolate as well as typical bodybuilding exercises, and by having so many body resistance (push-ups and pull-ups) exercises, the potential for building isn't as high. Body resistance exercises end up using many muscle groups, for example a chin-up accesses the biceps and back, where a standard biceps curl isolates just the one muscle group. So basically, doing a chest press on the bench is probably more effective at building muscles than doing push-ups. That is not to say that if you do push-ups to failure, you won't see decent results, it's just a fact of how things work!
As far as how to perform the workouts, picking the correct weight is of utmost importance. P90x blogger Steve Edwards suggested a kind of "phasing" overtime. For example, start phase I in the 8-12 range, and eventually get to just be able to do 6-8 reps by phase III. That means, literally pick a weight that will make you struggle even to get 6 out! It's ok to take more time in between workout to ensure you can complete them. And of course, build is in the recovery: drink your Results and Recovery formula and don't workout the same muscle group again for 48 hours. You may also decide to put Back and Biceps (pull-up heavy workout) on day 1, and Chest Shoulders and Triceps to day 3, as to not have another pull-up day so close to Legs and Back (typically on Day 5).
The p90x diet is kind of the reverse of a bodybuilding diet: cut first, than add the carbs for energy. To build adequate muscle you need to eat at least 1g of protein per pound of body weight daily (even as much as 1.5g) and you need to up your calories. You will gain weight as both fat and muscle as you workout hard and up your calories. Having a high protein breakfast in the morning is highly suggested. Using a Whey Protein supplement is also suggested, to help add clean protein calories. When you get to a desirable muscle mass, you can then decide to "cut" using a modified ketosis diet, or the phase I fat shredder diet. It's also suggested to do cardio first thing in the morning without having eaten during a cut phase, so doing p90x doubles during a cut phase would be smart. Other bodybuilding diet tips are to have your recovery formula immediately after the workout (40g simple carbs, 10g whey protein) and then an hour later, you can eat your "big" meal that has healthy fats, protein (of course), and complex carbs.
Ultimately, if you want to do a true bodybuilding routine, you will need even more isolation than p90x can offer. For example: Back on Monday, Chest on Tuesday, Arms on Thursday, Shoulders on Saturday, Legs on Sunday. There isn't any way to do that with the p90x DVD's. If bodybuilding is for you, talk to a personal trainer, or look at Body Beast, but if you like p90x and want to up your mass, focus on form, up your calories, eat your protein, and up your weights. It's that simple!
My P90X results were sort of the more "common" p90x look - toned with the makings of a six pack. (see below)
But, as you can see, I do have some nice muscle definition in my arms:
If you have some more questions, hit the contact page! I would be glad to help you customize your muscle building plan!