There are a lot of good points being sent your way here (you did come to the right place, after all
); I'll just reinforce a couple, with maybe a new thought here and there.
1 Staying on the offensive:
Very important. There are an unlimited number of questions or accusations that can be flung your way which will take much more effort to answer than to ask. When this happens answer one or two well, then segue into a counterattack which should be the main point of your talking segment. Or if you don't answer the questions at all and are challenged on the point, ask why he doesn't answer (one of the nearly unlimited questions that point out a huge hole in Creationism)? "After all, who's the teacher here?" may be slyly inserted, if you feel it appropriate at that time.
2 Know what you know:
You should have a supply of stock responses down cold. Read the source materials and practice speaking them confidently and with only the refrence you'll have at the debate. To prepare for pressure, try running around the block and then speaking through them, or something like that if you have time.
3 Be a rules stickler:
If they break the rules of the debate, call them on it. They may not stop breaking their own rules, but you'll score big points with your target questioning students anyway. Conversely it's very important that you not break the rules.
4 Take notes while the other guy is talking:
Debate fundamental. This helps to plan your response and keep track of what you need to respond to, and makes sure you don't get thrown off the scent by some throw away line or challange at the end of his statement. If he tries that out, start out with something like "Responding to the body of your statement..." to point out his misdirection to the audience. Then get on with point 1.
Your God is an asshole:
(If the debate turns this way, don't overtly force it or else you'll look like an ugly evangelical Athiest and their minds will snap shut)
This is the weakest point of conservative Christianity; it is certainly where I started to question mine. What kind of God condemns 90+% of all human beings who lived to Hell? Inefficient, at least. 10% is a pretty lame success rate for The Perfect Being. (If non-YECers are Hellbound the success rate will, of course, be much lower.)
"Are you trying to tell me God is some sort of demented cosmic third grader?" you might well ask of his Grand Game of Gotcha. Creating the Earth 6,000 years ago and then going to all the trouble of making photons already on their way to Earth from stars (apparently) billions of light-years away, creating the Cosmic Microwave Background Radiation to trick people into believing in the Big Bang and damning themselves, making a fossil record over a billion years deep and then planting it in the ground for scientists to discover, making Sol's spectrum read as a six billion year old main sequence star... well, it's an awful lot of trouble to go to just to "test our faith" or "get an excuse to torture lots of people forever and ever." And you want us to worship this psychopath?
Plenty of biblical examples too.