• TRANSITION BAG - for gear back pack style is a good choice since you may push or ride bike to the race staging area
• BIKE - make sure your bike is ready to ride, have your bike checked out at local bike shop to make sure working well.
• BIKE SHOES
• SOCKS - extra pair if rain predicted
• BIKE HELMET
• BIKE GLOVES
• SUNGLASSES
• BIKE PUMP -  which you can operate; If not get the bike support to do your tires DO NOT RIDE ARE UNPUMPED TIRES
• EXTRA TUBE - for bike tire (never know when you go to pump up tire at event you may have developed a problem w/tube)
• AIR CARTRIDGES - in the event of a flat.
• BIKE PACK - with items/instructions for changing your tire
• TRANSITION AREA TOWEL - for your space (extra towel if raining  & extra towel in car to wipe down wet bike)
• PLASTIC TRASH BAG - for the ground if it has rained (always keep one in your gear bag)
• WETSUIT - if using - extra bag for used wetsuit.
• BODY GLIDE - for chafing area
• Cooking Spray - such as PAM or Wesson; now there is a Wetsuit spray - optional and personal choice but make a choice
• BABY POWDER - (unscented/cornstarch type) to use inside your socks, they go on easier
• NUTRITION - gels, food bars such as power bars; rice krispie treats are also good but practice first
• SMALL PLASTIC BAG FOR TRASH
• SWIM CAP - Races give you one; Remember to put in your gear bag if you pickup your things before race day.
• GOGGLES - bring spare pair) (good idea to wash them off the night before/careful of applying sunscreen or moisturizer on race morning in goggle area)  I carry my goggles in swim cap to start not get them icky)
• RUNNING SHOES w/elastic laces - you will appreciate these laces; practice running with them first.
• MORE SOCKS if needed/optional
• HEADACHE PILLS use small Ziploc bag
• STOMACH PILLS or chewables - good idea to take something like Maalox or Imodium before the race if you get stomach issues
• 2 WATER BOTTLES for bike - fill with your sports drink of choice
• EXTRA WATER BOTTLE FOR HYDRATION - use water or sports drink before the race
• EXTRA WATER BOTTLE FOR PERSONAL USE - use to squirt the sand off feet
• SUNSCREEN/lip balm w/spf factor - waterproof sunscreen
• RUNNING CLOTHES - if you chose but best to do the race in the same clothing. Just deal with it!
• CYCLING CLOTHES - may be needed if weather changes or cool; such as close fitting windbreaker
• BASIN/PAN - for water to rinse feet/optional
• HAT/VISOR/BANDANA - for the run
• RACE BELT OR PINS - for Race Number
• If racking your bike the night before, you should have trash bags and small plastic bags to cover your bike components overnight.  If this sport becomes your hobby, invest in a plastic bike cover.
• DRY CLOTHES - for after the race
• COMFY FLIP-FLOPS - your feet will thank you

One of my mentors provided me this list early on and I have forwarded it on to others many times.  It's by no means exhaustive, but is a great reference for people who are just getting started in the sport.  Please feel free to make any suggestions for updates.  I would love to clean this up over time and hope it finds good use by others.

• TRANSITION BAG - for gear (back pack style is a good choice since you may push or ride bike to the race staging area)
• BIKE
  

I made this checklist before my first 70.3. The left side is my list. The right side in the colors is my transition layout. The bottom right is actually the order that I did everything when I was in transition. I didn't look at it during the race, but had it with me pre-race and studied/practiced my order over and over. This way on race morning I didn't have to think about anything, just had to look at the sheet to remind me of everything.