Pack up the night before the race. As much as possible have the car ready to go. You will have to get up very early because you want to get there as early as possible to set up transition. Being ready to walk out the door will allow you to sleep as late as possible in the morning.
I use a checklist to make sure I remember everything. Below is a checklist of whatever I could imagine to get you thinking. Toss out what doesn't apply to your needs. But have a checklist and use it.
In the list below you'll see an item called "Large Colorful Towel." I place that on the ground underneath my gear. That makes it easy to spot my stuff. This is not that difficult for T1 because my bike is there and it's bright yellow and sticks out like a sore thumb. But for T2 you can't look for your bike. You've been riding it. It's in your hands. Plus the transition area will be starting to look it's been hit with a bomb. Things are scattered all over the place. I spot my towel, and I'm home free.
Something to Eat while Driving to Race
Water for Drive to Race
Race Drink for t minus 30 minutes
Backpack for Gear
Plastic Bags for getting into wet suit
Large Colorful Towel
Number Holder & #
Number for Bike
Replacement Tire Tube & Changers
Air Pump (for getting ready)
Air Cartridge (for the race)
Visor Running Hat
Arm Sleeves (spf protection)
Maps - Registration Materials
Clothes to change into afterwards
Shampoo & Brush
Food for after race
Cooler with Drink for after race
Folding Camp Stool
Warm Tech Shirt