I'm not sure about Division II or III schools. The D III school I coached at in southern Illinois for a couple of years didn't hold open tryouts. But we did at Division 1 Bradley University, where I played my college ball.
I think the thing about tryouts is to stay positive. Stay serious. And work hard. Do things that make yourself stand out -- hustle, poise, etc.
Also: dress the part! I can't tell you how many guys showed up in jeans for the tryout. Don't do that! Wear sweat pants or baseball pants, sleeves (don't do the muscle-T thing), and a hat. This might seem like "DUH!" but you'd be surprised ho many well-meaning kids don't give themselves a shot at making the team simply because of how they're dressed.