Everything begins with the "core" muscles. If these do not work properly, nothing else will.
Start with strengthening the abdominal muscles: pilates core exercises are ideal. The goal is "ab-awareness."
Also, work on the "back" muscles: those that hold the shoulder blades back and down: rowing machine exercises are excellent. The "front" (e.g., pectoral) muscles need relatively less attention, since we habitually exercise them already.
Focus on the large muscles of the body, and the smaller ones will take care of themselves.
Beware of rigidity. A good frame is flexible and, for the lady, one that enables her to "fit the man."
Few things in life are more fun than dancing with a lady who knows how to "frame-up" (as in "this is a frame-up")

Good luck!