The Gemara Marking System is an innovative method that uses geometric shapes to structure the Gemara, thereby increasing understanding, retention, and effective chazara. Over 25 years in development, this method is currently being used by thousands of people worldwide.
This web site has a number of audio shiurim and sample marked pages to teach the system, as well as a continually updated Daf Yomi archive of both audio and scanned marked pages, starting in Bava Basra.
I used this system to learn through the entire Shas, and I can honestly say that it would otherwise not have been possible! The website has a great introduction where you can learn how to do it yourself.