make a list of everything you will need, include even the obvious things, like your bike.
do this a few days before the event and leave it on your table so you can add things to it as the race approaches. there is nothing worse than forgetting something important on race day.

Arrive early to the transition area, even earlier than you feel you need too. I pretty much always get there when it opens. This will give you time to get all your equipment laid out the way you want, time to stretch, warm up, calm your nerves, etc.

Make your bottles the night before and get your nutrition plan decided long before race day. Oly races are long enough that you will need to take something in other than water. Obviously you should plan your own based on what works for you in training, but here is what I use to get you started. I start the morning off 2-3 hours before the race with a breakfast, usually bagel, hard boiled egg and juice. 30 minutes before the swim I eat half to 3/4 a package of clif bloks, the rest I put in my nutrition pouch behind my headtube. On the bike as soon as I am away from the transition I drink 6-12 ounces of sports drink, usually something with a bit of protein and chase with equal water. At the turn around I take in the rest of those clif bloks and chase with water. On the way back I take a small drink every 10 - 15 minutes alternating between a sports drink and water. (FYI I should have mentioned this earlier but I carry two bottles behind my seat and an PF aero bottle between my bars, I recommend bottles behind the seat because it forces me to stretch which helps my run.)
As I start the run I take down one gel with caffeine the second I see the first aid station. When I hit the aid station I always take a water to help the gel process. I rest of the run its just water over the head. It's only 6 miles and the first aid station is usually at mile 1 so it's not bad. This is my plan, like I said build your own but this could give you some ideas. Nutrition is very important to a race of this distance especially if you have only done Sprints before.

Key to racing, never try anything new on race day. Oh and buy a race belt before you get there. They will sell out at packet P/U.

Small tips for actually racing, I don't know how strong of a swimmer you are, but up until this season on beach starts I always ran to the outside of the pack. There is nothing worse than taking a kick in the face 100m in to a race and being disoriented for the next 1400. That extra minute or two you spend in the water staying away from the crowd wrestling each other will be made up three times over on the bike with the energy you saved.
On the bike, focus on your pacing, 24 miles isn't much but remember you don't want to go anearobic (sp??) too early and be dead on the run. For the run, I set my pace to be about a minute/mile slower than my average 10k time.

Transition area tips - Most people don't wear socks on the bike, I personally have to, the design of my feet demand I wear socks so I always set a towel on the ground just in front of my bike, this allows me to dry my feet without having to bend over and pick up the towel. Don't try putting cycling socks on wet feet, it doesn't work. If this is your first race of this distance I do not recommend leaving your shoes on your bike, if you do feel this is for you then there are some good forums where we discuss stragety on this (just do a search for the topic).

last and most important piece of advice, Have fun! That's what it's all about.