With Tristan's slow climb back to fitness, I've been working hardest on making sure he's using his back effectively. I don't have a great natural feel for how a horse is moving underneath me - in fact, I have zero natural feel. Everything I have has been drilled into me by many frustrated trainers.
So I'm always looking for ways to learn more. A friend on Facebook linked to this article, which does a good job of describing what I'm looking for but the real gold is in a video the article linked to. It's really outstanding. It's given me things to think about and a much more clear visual reference for what to look for. (I probably could've picked these things out before but I don't think I could have really listed off why I thought a particular horse was better through its back.)
Here it is embedded for reference.