One thing you are bound to notice at any debate is the different speaking styles used by the competitors. Speaking style is perhaps one of the most difficult aspects of debating to attempt to "teach" as the best style for someone is something that develops naturally over time. However there are a couple of things to be kept in mind.
1. You must speak clearly and loudly enough so that your voice can be heard by everyone. Remember the adjudicators will sit towards the rear of the hall so at the very least they must be able to hear what you are saying if you are to have any chance of winning. However you shouldn't shout as the halls have generally been designed so that your voice will carry towards the back.
2. Try to avoid monotone. If you are making an important point use your voice to stress it and make it stand out. Try to slowly increase the stress and force behind your voice as you go through your speech. Build up to a high point and make this the crucial point of your speech. However don't bring the audience on a rollercoaster ride. Don't start high, fall down, build-up and fall down again, it looks as though you are only convinced about the truth of half your speech.
3. Keep eye-contact with the audience and don't stare at the podium. It gets easier to do this after some experience and once you use fewer notes. Some people like to pick out individuals in the audience and look at them. Others just speak to the audience as a whole. However you do it make sure to scan the audience and move your gaze to different parts of the hall regularly.
4. Use your body language to back up your speech. If you stand rigidly and don't move then you will find it very difficult to have any real conviction in your voice. Use your arms and facial expressions to convey your emotions and back up your speech. However don't go overboard, you want the audience's attention to be focused on your speech not your arms. Try not to have anything in your hands. Some people like to carry a pen and end up waving it about like a baton which can distract the adjudicators. If you really need something use index cards.
5. You don't have to stand strictly behind the podium. Move around a bit and face different sections of the audience at different times. Apparently studies have shown that people tend to prefer to be able to see the whole person as this is supposed to indicate that you aren't hiding anything. However, once again, don't go overboard. It annoys people (and more importantly adjudicators) if you walk too far from the podium. Try not to go more than 1-2 meters away from the podium. One way to ensure this is to leave your notes on the podium, you'll find yourself reluctant to move too far from them.
6. Don't be too complicated. If your argument is too elaborate people may have difficulty following it. Don't use 15 syllable Latin words when a 2 syllable English word will do. Remember you are trying to convince the audience that your argument is the best and not that you consider your talent wasted on them (even if it is).
7 . Use humour to help win over the audience and make your speech stand out. If you have a natural talent for comedy or impersonations etc. then use it. If you don't then don't worry about it, even the most serious of us can be funny at times (often even without meaning it). You can work out a few put downs and one-liners in advance but be careful. If a joke sounds too prepared than it may bomb. Try to make it sound spontaneous and it's more likely to be successful.
The best thing to do is watch other speakers and see how they combine the various elements. Experiment with different styles and try to find one that you are comfortable with. However the only real way to develop a good style is to try to speak on a regular basis and listen to the advice of adjudicators and the more experienced debaters.