This is an exercise I recently started working on, and I'm very happy with the results so far. I'm always looking for ways to make smoother phrases. This means we must avoid slamming down the keys, and we have to keep the air supported as the notes change. I call this "the air goes zoom."
Play a middle G, and focus on relaxed fingers and fast moving air. As you lift your ring finger to play an A, imagine that the finger is being blown upward by your air. Repeat this process, slowly moving to a B. In the beginning, you will probably slightly "puff" the air as the fingers move. Once this is accomplished without effort, imagine that the air column is constantly pressurized, and that simply relaxing a finger will cause that key to pop up. At first, only work on finger exchanges that "release" keys. Once this is mastered, try imaging the air flowing into your fingers to gently pop the keys downward.
Work through the exercise slowly, and really internalize the feeling that you are controlling the fingers with a rapidly moving, highly pressurized column of air. Imagine that the keys are pneumatic, and that the fingers mostly get out of the way of the air. Try to feel your diaphragm pushing at the keys. Feel the air moving under your fingers, even through your fingers.
In time, the fingers will become increasingly relaxed, resulting in a fast and nimble technique. Keep the potential energy of the springs in mind, and avoid squeezing or clamping down. Whenever there are technical problems, first move your attention to the airstream. Imagine the air rushing through the instrument, literally powering the music. Each time you imagine this, think the phrase, "the air goes ZOOM."