I think the beginners you see sitting the trot are sitting a slow, shortened trot, rather than a true dressage working trot. And they're probably riding schoolies who have learned not to move through their backs and only move their legs in self defense, so they're quite easy to sit, though incorrect.
To add to what Spyder has already said; I would say that when you can maintain a good following seat with adequate motion in your seat bones and hips at a strong working trot WITHOUT actively concentrating on it, without using a "sissy strap" and you can apply other aids without losing your following seat, then you're reading to ride sitting consistently.
For me, it took A LOT of time on a lunge line on a big moving horse before it became automatic, and I didn't have to think about it all the time.
Last edited by maura; 01-01-2011 at 10:22 PM.