Sit or stand?

I think you should work on standing more. I ride single track and trails too and notice that there are lots of whoops (in the sandy sections), roots, and of course sand. The best way to negotiate all of these obstacles are by standing. Granted, some trails are tighter than others and this will require sitting (as well as hairpin turns around trees). But, standing offers you the ability to move around and balance the bike better, and when you deflect off of one of those roots -- the ability to correct and if need arise, to dab your foot at the ground to prevent total loss of control. You are also able to see farther down the trail and even the ability to look across the woods to a parallel trail you may be negotiating around the next turn. You have to be moving constantly though. I'm constantly ducking branches and using body english to avoid the deep ruts and large roots that develop in the overly used sections as well as those instances where you have to squeeze between trees (I've found myself actually leaning against the trees as I maneuver my bike between them -- something I can't do while sitting.). It just takes lots of practice. I go to my local riding area and practice chasing people through the woods. I try and stand the entire time and notice I stick to their back tire a lot better and don't get "out of shape" as often as they do. It's just smoother. If you want to get your body used to it quicker, try strengthing exercises for your lower back and quads. (It'll help you transition from standing to sitting to standing and leaning over your bars (something you should do while standing to help you to better grip the bike with your legs and make the bike easier to hold on to and maneuver when your "bursting" through the faster sections.). I hope this helps and am sure you'll probably get a lot more information from more qualified riders than me! Happy trails!