Preparing a packing list for your triathlon/duathlon is one of the most important things you can do before race day. Some key items to include are: USAT license, photo ID, race outfit, swim goggles, swim cap (give to you at the race), towel, bike, helmet, sunglasses, bike shoes, water bottles, nutrition (gels, bars, etc.), and running shoes. You may also want to bring: money, wetsuit, socks, and change of clothes. http://triathlon.racechecklist.com/ is a great interactive resource that allows you to create and print your own packing list.
Here are some other sites that might help you decide on the gear you want to have for race day: