Unfortunately for many students involved in the activity, debate is more than just about who is the smartest, does the most research, or can speak the fastest. Because debate is a very subjective activity, there are many other factors that may influence a judge's decision, especially in rounds that are close.
How to dress
Contrary to popular belief, most judges like to see students dress up to some extent for debate tournaments. It signifies respect for the activity and for the coaches and judges who invest so much time and energy into debate. This is not to say that men should wear three-piece suits or that women should wear expensive elegant dresses to every tournament. Just dress in a neat casual way like dark khakis or slacks. Also avoid other common mistakes such as the just woke up hairdo or really bright, blinding colors. Part of public speaking is looking nice, however, you should make a decision as to where you feel the most comfortable.
Body language also speaks enormously about your respect for the judge and the debate round itself. Try not to slouch, hug the podium, roll your eyes, move around, or make wild hand gestures while speaking. Stand straight up while making eye contact with the judge whenever possible. This is not meaning to stare at the judge like a statue or stalker just remember they are the ones you must convince to win. Excessive fidgeting and poor posture are signs of immaturity and are not appealing to "grown-ups." Keep you head up when your speaking or questioning. Looking at the ground makes it harder to hear you and shows that your intimidated or nervousness.
Having a Strong Presence
Presence in a debate round is defined by two important factors: confidence and charisma. Both are hard to develop but can be extraordinarily effective. Confidence should be a natural outgrowth of experience and research - the more you know about the debate topic and debate in general, the more likely you are to feel comfortable in a debate round. Charisma is a gift that not everyone is blessed with. It involves the ability to persuade people, to strike a chord with the judge on almost any issue. It is not some magical attribute but a complex skill that is developed with experience and partly by mimicking speakers who are charismatic. This is one of the hardest skills to learn for some while others have it naturally.
A horribly underrated virtue in debate is politeness. While rudeness, anger, and hostility have their place in debate, and some of the most fun parts, they are strategic tools that should be used only when appropriate. Judges tend to tire of empty slurs fairly quickly, and are far more likely to be impressed with individuals who intimidate others through superior skill than they are with debaters with a large assortment of insults and sly remarks. Remember that they way you treat everyone is important - not just the opposing team. Respect your partner, the judge, time keeper, and any other people in the audience as well.