Blankets for Thoroughbreds

From time to time our readers ask us questions. Here is one - maybe you as readers can offer suggestions: This question is posted on our forum and the facebook fan page:

Hello Deb.  On behalf of  owners training thoroughbreds as dressage horses (or as anything else!), I have a question.  What makes of blanket or summer sheet will usually fit a thoroughbred properly around the shoulder?    From my experience, they usually hang sadly from somewhere close to the wither area and end up slithering backwards until the horse must feel it's in a straight-jacket! 

Any help or suggestion would be greatly appreciated by me, my horse and other thoroughbred owners I know! 

Janet