I'm not sure what I can say about building stamina. For one thing, the bar will obviously increase it by lots, but you're probably already using that. Another way is to move around less, if that makes sense. You don't have to lift too far off of the pad, and you don't have to bring your entire foot to the arrow that you're pressing. Probably nothing you haven't heard, but still something to keep in mind.
Of course, no matter how you play, endurance will build up in time. Believe me I remember being at the "tryin'a pass Hardcore Symphony" level, and how quickly I grew out of it. You'll be passing twelves by the end of the year even if you don't play more than once a week. Just push it to the limit on your last song every time.