In addition, if you are just beginning, go to a local music store and pick up a beginning band book. This will have a fingering chart of all of the slide positions and will talk about how to truly form the embouchure and have pictures of what the correct embouchure will look like. Also, many books these days will have DVD's of someone playing. This will give you an idea of what the characteristic tone of the trombone should be. Also, don't forget to learn how to tongue (articulate) on the instrument. On trombone, you always use your tongue. Of course, if you don't, you will be playing with all kinds of glisses in between the notes. (This is a pet peeve of mine.)
If you are past the beginning stage, then I would try to listen to as many trombone players as I possibly could. Now, the problem is, there arn't that many recordings of solo gospel trombone players. Therefore, you will need to be somewhat of a trailblazer. I would start by listening to the great jazz trombone players: J.J. Johnson, Bill Waltrous, Rob McConnell, etc. just to get an idea of what the instrument can do. Afterwards, I would listen to other instrumentalist like sax players playing gospel, Rick Braun on Trumpet, and whoever else I could listen to. Take what you hear from them and see if you can apply any of it to the situation you are playing in.
Above all, remember that music is an aural tradition. Listen as much as possible. Transcribe solos and buy as many CD's as you can. Start a collection of "licks" that you can use when you solo.
Hope that this helps somewhat, and good luck on your musical journey. I await the first ever gospel jazz trombone CD.
