Principle of Overload - Training at just a bit more than your body can normally do is a good idea, because that allows your body to adapt to it, so that when you come back to that workout that was once too hard, it'll be a breeze. Eg. it's like when you're playing an RPG and you fight guys that are a higher level than you because they give higher experience than lower level guys
Principle of Progression - Work your way up when you exercise. If you start way too high, then you're obviously gonna crash. Eg. You should fight enemies that are a slightly higher level than you, but don't fight the final boss at the beginning of the game unless you can handle it!
Principle of Specificity - This doesn't apply a huge amount to p90x, but if it were to apply, then I'd say that if there was a part of your body that you really wanted to train, then specify the workouts to that. But, in p90x, it might be a better idea to go for the whole body. Eg. If you want to get better at sword mastery, fight the enemies with a sword and not a boomerang!
Principle of Reversibility - Make sure that you keep being active after you finish your training plan like p90x, because after a while, the changes that happened in your body from the workouts will eventually reverse themselves back to the way you were before you worked out, resulting in a big waste of time. Eg. In something such as shoddybattle, you can work yourself up to a humongous rating, but if you stop playing, it'll go back down to what it was when you started
Principle of Diminishing Returns - After training for a very long time, it'll get much harder to get changes in your body. You will notice a multitude of changes in your body for the first couple of months, but after a while, those changes will be much start coming at much smaller rates than when they started out. Eg. The higher your level, the more experience you need to level up, and therefore the enemies must be that much harder if you want to level up.
Principle of Individual Differences - Everyone is different, and therefore all changes happen at a different rate for everyone, so don't be disappointed if your changes happen later than your friend's, and likewise, don't brag if your changes start happening earlier than your friend's, because they will come eventually if you keep up with the training! Eg. All characters level up at different rates.