After I said, "practice", I asked him if he listens to a lot of music. He said, "not really."
Now, I will admit that I wasn't surprised to find a young player that doesn't listen to a lot of trombone recordings. However, I was surprised that someone who doesn't listen much did have the curiosity to ask the question in the first place.
It is ABSOLUTELY ESSENTIAL that you, as an aspiring musician, listen to as muc music as your ears and brain can stand. Preferably music that is directly related to the styles in which you want to be fluent. If you play the trombone, and you tell me you've never heard a recording by Joe Alessi, Christian Lindberg, or JJ Johnson, then you have some serious work to do! I discussed with the young man where he might go to hear great performers: YouTube, iTunes, Amazon, Spotify, etc., and most of it for free!
The fact of the matter is, you will play the sounds you have in your head. If you can hear it, then you can proceed towards reproducing that sound.
Second, you should be listening to music that you might think doesn't interest you. Over the past two weeks, I did two things that were eye openers for me: I toured the Moog synthesizer factory in Asheville, NC, and I heard the Blind Boys of Alabama while we were in Birmingham. The Moog tour was interesting because our tour guide, besides giving us a great overview of the history of Moog, pointed me towards some fantastic musicians and bands whose work I discovered I really enjoy, such as Deltron 3030 and Dan the Automator. Those are bands and music styles I would have NEVER discovered on my own! Likewise, the Blind Boys of Alabama do play a style of music that I already enjoy, gospel, but their live show was so much more than that. Backed by an absolutely fantastic 4-piece band, they played gospel, soul, country, rock, and put on a live show that was as entertaining as it was musically amazing!
So, listen, listen, listen. No excuses! That is, IF you want to improve and broaden your horizons.... :)