I too would say Heather Moffett teaches the best way I've found so far. I have her book "Enlightened Equitation" and the analogies she uses helps me out. My RI also advises me to lean back as far as I can until I can feel my stomach muscles working. Given that I used to ride (and am still prone to) in the fetal position, this helps get me upright, but also helps me absorb some of the movement. I also need to remember to keep my shoulders back, as this also helps keep me upright, and allows my core muscles to go with the movement. The absorbtion techniques are very similar (to my knowledge) for canter and sitting trot, so being able to absorb the movement in one will help the other.