There are a wide variety of exercises you can do to help tone and strengthen your back, legs and stomach in preparation for riding. When beginning these exercises, it's important to learn how to do them correctly to avoid injury.
Rather than trying to explain the exercises in detail, I'm going to point you toward some online and offline reading resources written by professionals.
Fitness Tip of the Month - Improve Core Strength
Strengthen your lower abdominals and core strength with these rider fitness exercises.
Pilates for Riders
Learn why the Pilates exercise system can benefit your riding both physically and mentally, and how to choose an instructor.
The Rider's Fitness Program
Available online at HorseBooksEtc.com, this six week workout routine is designed especially for equestrians.
Better Rider, Better Ride
Available online at HorsebooksEtc.com, this book offers a complete program to help you assess and improve your fitness level and become a better rider.
Pilates for the Dressage Rider
The goals of a Pilates program are often identical to what riders try to achieve in the sport of dressage. Available online at HorseBooksEtc.com, this book by Janice Dulak contains exercises specifically chosen to best suit the needs of the dressage rider.