There is a book by Chase Sanborn called "Tuning Tactics". The CD that accompanies the book has recorded drones(Synthesizer playing pitches) in all the keys. What I do in my practice is play scales, chords, intervals,jazz licks etc all around the center key/pitch that the drone is playing. Its a very meditative experience once you get used to it. This is very helpful in getting the clarity or vision of sound inside your head and consequently results in a more vibrant, clear, focused sound. I also use the drones to practice "lip bending" as has been described by others. The drone again is helpfull in developing your sense of pitch center and that is what eventually will develop the tone you are looking for.