One thing that has always made a big difference for me during a plateau is to completely change the workout for a couple weeks. If you’re doing 8-12 reps, go 15-20 with 60 sec rest between sets. Change what exercises you are doing too. Cardio is the same way, if you’re running distance all the time or just doing HIIT, your body will adapt. The P90X guy is absolutely correct with muscle confusion. I have to change my workout ever 5-6 weeks with 2 weeks of shock workouts to rev my body up. The other thing is that you may need a break. Your body and CNS can become fatigued with long-term workouts. Every 6-8 weeks I take 3-4 days off, completely off. I may go for a walk, but I’ll just enjoy my free time. This will relax you and you will come back really fresh. Just make sure you adjust your diet to completely eliminate fast-acting carbs during that time as your insulin sensitivity will change.
I am also in a big time plateau, so I feel your pain. I can’t gain weight to save my life and my lifts have stalled because of it. Opposite end of the spectrum, but we all have our times where we’re just stuck.