Bascially all P90x is, is a program that is charging you money in exchange for telling you what muscle confusion is.
Basically, if you want results just change up your workout every 6-8 weeks
I'd have to disagree. Simply 'changing' your routine isn't going to achieve amazing results. Most people see, "change your routine" think of it as instead of doing Leg Press, Hack Squats, etc etc. I will do an extra set and do DB Squats, or Romanian Deadlifts with DB instead of BB. It will help, don't get me wrong. But muscle confusion is much deeper.
Muscle confusion to a sense isn't exactly just that. Muscle confusion has a lot to do with changing up the pace, rep-range, and all the other intricacies in the bodybuilding/physical fitness realm.
For instance, instead of training the conventional Chest/Tricep day. They pair up Chest/Back..opposing muscle groups. I personally love the split set-up as I have achieved great results.
If you don't want to do p90x I would suggest doing a circuits in a gym. I do 2 exercises with rather heavy weight and will do either a bodyweight exercise (focusing on core movement), jumprope, or a specific ab exercise.
I'd also suggest getting a heart-rate monitor (watch your breathing), and try and keep a sustained heart of 140-150ish (thats a range that feels good to me, everyone WILL be different).
Best of luck.