Hilda Gurney is one of the greats of American dressage. Here’s a quick (3 minute) video overview of correct rider position, filmed during the 2012 USDF Annual Convention and Symposium.
Correct rider position with Hilda Gurney
And here’s a good rider body-awareness article — it’s a .pdf file from USDF e-Trak: http://www.usdf.org/EduDocs/The-Rider/ABodyAwarenessPrimerforRiders_2008_Nov.pdf