At my peak while riding my MTB, I would rarely ever stand (I am 6'1" ,~220) I learned to relax my upper body, and just focus on turning the pedals, trying to maintain a smooth cadence. I only stood if I needed to power over a small hill, or obstacle. And I usually out climbed my riding partners.
I am working on getting back into that shape now. Doing elliptical and weight training when I am not riding.