You need to know what you want your sound to be like. That is the straight simple answer. If you do then you will strive towards that ideal. There are ways to practice too. Long tones and the overtone singing method that Sabutin talks about could help to find and improve a better tone, but for that to work you need a lot of skills to begin with. You need to know what to listen to and how to control the resonance. You need to experiment with your mouth cavaty and before that you need a working emboushure. It is not easy to describe how to improve your sound. It needs serious work. Listen to tromboneplayers you like and try to imitate the sound you hear. You could also record yourself and listen. The sound consists of so many things. Sound is articulation, dynamics as well as vibrato, the whole lot. Personally I try to play as musically as I possibly can. I record myself and imagine someone else is playing. This way I take a step back when I'm listening so I can listen more objective.
/Tom

