Pack your race packet, money for fees and a photo ID.
Pack your swim stuff: bathing suit, goggles, cap, wet suit.
Pack your bike stuff: bike (make sure tires have air), pump, water bottle, repair kit, bike gloves, shorts, shirt, helmet (cannot compete without this), bike shoes, socks and some sort of brightly colored item to tie onto the bike.
Pack your pump-me-up music for the drive down.
FYI - It is poor triathlon etiquette to listen to headphones during the run and it is absolutely prohibited during the bike leg for safety reasons.
Have a healthy dinner including carbs.
Set your alarm clock so that you have enough time to eat breakfast, go though your checklist one more time, get there and still have an hour before the race.
Go to bed early! You need all of the rest you can get!
Race Day - Night Before the Triathlon
Swim
Bike
Run
