I agree with DuffyDuck. Read everything you can in dressage theory. I was told you should spend 2 hours learning theory for every hour in the saddle.
It's important to understand why we do the stuff we do at training level. These are the basics, the foundation that every single piece of dressage depends on. If you just got through the motions without understanding how it all fits together at the end, then you won't progress much past training level.
The other thing... The training pyramid is not a stack. It's more like a circle. You need all of the parts at every level, but in varying degree's. Even at training level... you start with the very bottom (rhythm) while warming up and when that's ok, add in suppleness and contact (etc) diring the ride. It'll be at a very, very low level as you start. As your training progresses, you'll find that you've improved on each part. But you need all of it to move up through the levels.
I've never owned a horse that I didn't start from the ground up in dressage. Occasional I get a chance to ride a horse a little better trained, but its rare. It's harder to do it this way. If you have an opportunity to ride a horse that's working at a level higher than you, then jump on it. No amount of lessons or reading can replace the "feel" you develop when you've sat on a horse that knows his job. Posted via Mobile Device
Presently I am reading a book entitled "Feeling Dressage" and it has helped me a lot. I found out that if you know the theory it is much easier troubleshooting and making it correct. Right now I am trying to understand travers. How to do it? What is the relation of travers with other exercises such as half pass? What do I need to do correctly before trying travers and so on... If any of you have any suggestion of reading or freely available internet movies I would appretiate.