Not exactly ground breaking...but good advice.
During workouts, try doing your usual standing exercises on one foot or with one or both feet on the Bosu balance trainer - either ball side up or down (both feet).
E.g.: Curls, shoulder presses, lateral raises, overhead tri xtensions, etc. Switch legs halfway through the set.
For more challenge / abs, lift the "up" knee up and keep up thigh parallelle to floor. For your butt and hip aductors, hold one leg out to the side. (Owwwwww -it burns!)