Work on the actual jumping motion. It is a common tendency for younger players to focus almost exclusively on the upper body motion and the results at the basket. However, also pay attention to what your lower body is doing, as this has just as much effect on your shot as what your arms do. Bending your knees and actually jumping is a key action. This will help you to obtain a good arc on the shot.
Practice your release. If you are right handed, the left hand is only a guide, nothing more. Many young players tend to shoot with both hands, which negatively alters the shot. Even the pros make a habit of fine tuning their releases, as evidenced by Lakers great Kobe Bryant once hiring former NBA sharp shooter Chuck Person to help him with his release. The key is to follow through with your shooting hand. Flick and bend your wrist all the way through, even when the ball is out of your hands.
Run at full speed, then stop to pretend you're making a jump shot. Do this over and over. It's one thing when you're by yourself in a gym or your back yard, but it's entirely another when you have one or two defenders hounding you. It's important to simulate real game situations where you'll have to stop and pop.
Find an alternate target, if possible. For example, a square drawn in chalk on the side of a building could work. Basically, anything to aim for is valuable as you hone your jumper without a basketball.