Increase the allowable air pressure inside a ball with a bladder, such as a football, basketball or soccer ball. The less the skin of the ball dents when it hits an object the greater the amount of original energy is available to provide a rebound. A ball that has more air will dent less than an equal ball with less air.
Harden the casing or the inside of the ball. For instance, wrapping a baseball tighter with its inner wool and polyester yarn or changing the outer cowhide to a harder leather will prevent the stored energy from the pitch from being lost while the ball flexes against the bat. The kinetic energy is available to use to accelerate the ball outward with higher velocity. A looser wrap or softer cover will create a "dead ball."
Change the surface of what the ball is bouncing against to make it harder. A rubber ball dropped on a carpeted floor doesn't bounce as well as one dropped on a wood floor because much of the kinetic energy has been lost to the effort it took to dent the carpet. The wood floor does not dent as much, allowing the energy to stay with the ball and rebound higher.