The following description is for a right handed person working on their left hand. Begin on the baseline near the right side of the lane. Simply dribble in a big circle near the boundary lines and finish with a left-handed layup. It's important that you are cutting back to the right throughout the circle. The act of cutting to the right, while dribbling left handed, teaches your hand to control the ball.
Smaller circles are also effective.
You can start on the block, dribble in a cirlcular pattern around the top of the key, then go in for the layup. Be sure to keep your head up as much as possible, or even use dribble glasses if you have them.