Play loudly to the point where you sound good. Any tips we could give you beyond that could be detrimental to your playing. Some mouthpieces have special baffles that speed up the air, and there are ways to sort of make baffles on the spot for something like a C* (essentially by molding a piece of gum or similar material to fit in the mouthpiece), but you also could be somewhat limited by your horn.
When I was in Middle School jazz band, I can still remember my band director egging me on into playing louder. He even challenged me and another playing to see if he was louder than us. In truth, he was playing on a professional horn and had much greater chops than my friend and I. To this day, I can get my pro horn waaaay louder than my student Yamaha, and still sound good. What I realize now was that he didn't want me to play louder, but bigger, which is an important distinction.
I think a big part of all of it is attitude too. You are performing, so show it off. Stand a little straight, pretend that you're the best sax player in the entire world and everyone wants to hear what you're about to play. You might be surprised that this will effect your sound, but it will.