Being vigilant for 10-12 hours on stand is difficult. I sat in my climbing tree stand one day and as I was coming down, and already had my gun on the ground, a nice buck showed himself. Nothing I could do but watch him trot off.
Being comfortable (warm) is job 1. When my feet start to get cold I just wok my toes back and forth until they start to wrm up. I also just put part of my weight on my legs, like I'm going to stand up and hold there for 20-30 seconds at a time, basically doing isometrics. You would be surprised how well it works to get the blood flowing.
When I go to Alberta, I will have one of these: http://www.heaterbodysuit.com/index.htm

Food is the second thing that seems to keep one from getting up and leaving.

If I do get up and walk around, I will rarely go back where I Sat in the morning.