This might look a little over the top, but I use this checklist now after forgetting a few things. However, there's nearly always someone around who's happy to lend you things if you forget.

Tire pressure gauge
Front and rear stand
Tire warmers
Tie downs and bungy cords
Locks for bike
Battery charger
Generator
Air compressor
Ramp

Oil
Gasoline and fuel can
Tools (sockets, wrenches, spare bolts & washers)
Zip ties
Cleaning liquids
Rags
Fold out chairs
Tent, sleeping bag(s) and ground cover
Rope
Canopy for shade
Buckets

Carpet to go under bike
Fire extinguisher (must have one per bike)
Paper and pen
Masking/painter's tape for grid position

Keys to bike
Boots
Gloves
Neck brace
Helmet
Racing suit
Tinted and regular visor
Ear plugs
Thermal undies and neck gator
Latex gloves for cold weather

Cameras and batteries
Sun cream
Baseball cap & sun glasses
Water & distilled water for radiator
Food
Cooler with drinks
Wet wipes
Phone & charger
Radio (to listen for announcements)