If I were you, I'd first do what one person here suggested, which is to work on your fundamentals as much as you can to improve your tone before you consider buying a new mouthpiece. This accomplishes a bunch of things: 1) It makes you a better player, 2) When you decide to buy a new mouthpiece, you'll have a better idea of what to look for, 3) Appeases the people who'll say you should just be good on what you have, or else. #2 is probably the most important in this case...although #1 should always be your goal!
Then, I'd go
here and read all five segments. I know I wish I had before I decided to change mouthpieces.
Then, after all that, go to your local music store and try as many mouthpieces as you can so that you can get an idea of what you like and what you don't like. Now, if you talk to a lot of people who "know" their stuff, they'll probably tell you a few specific mouthpieces to look for (like the Bach 5G or the Schilke 51). I'd take that advice with a grain of salt and give those mouthpieces a try, but also try other ones, both mouthpieces of the same brand but different sizes, or mouthpieces of other brands and similar (or different) sizes. Don't buy a mouthpiece without trying it!!!
Hmm...I'd recommend that you try mouthpieces from Schilke, Bach, Stork, Dennis Wick for sure (I know a number of people who swear by at least one of these) Other ones to look for include Greg Black (I know a bass player who looks down on anyone who doesn't use these), Edwards, Doug Elliot (My professor uses this one) and Griego. Also, make sure you try at least one gold mouthpiece. Their effect on tone isn't clear cut, but they definitely have a different feel from regular mouthpieces, and you may find that you like that feel better than regular ones (I know I did...but for some reason I ended up with a regular ol' nickel mouthpiece!)
Then, after you've narrowed it down to just a handful of mouthpieces that you like, take them all to someone who "knows" the trombone (like a professor or private teacher), and have him do a blind test of all of them (so that he doesn't know which one you're playing). That way, you'll know which one effects your sound in the best possible way.
You may have to repeat any of these steps a number of times. I currently play...well, a mouthpiece I like very much (I don't wanna say which one so as to make you think it's the best...), which I found after three stops at stores and playing through their mouthpieces, playing through all my professor's mouthpieces, and taking three mouthpieces from my local music store for a week and playing all three in a variety of environments, and then two blind tests with two different professors before settling on...well...the mouthpiece that was right for me. But I guarentee you, if you are even half as meticulous as I was, you'll be happy with what you finally do buy for a very, very long time.
Best of luck!
