I'm not a triathlon expert, but I've completed seven of them now (or possibly nine, if you count two practice events that were full sprints) and feel like they're no longer mysterious. After seeing Gina's post about tri tips, and knowing Erika is about to do her first tri this month, I thought I might share a few things I learned. I'm not going to go over what to do to train and get fit--you're already there. Instead, I'm going to share some tips that I think help me perform better on race day. Most of these are just little hacks that will save you seconds and frustrations.
Set up and run through your transitions at home
Completely lay out all of your gear--your running shoes, bike gear, bars and gels, and any other errata--on your towel or whatever you're going to use in your transition area. Now, put on all of your swim gear--including the wetsuit and goggles--and exit the room. Run in, strip off your wetsuit, and put on your run gear. Run out and back in again and do the same with the bike stuff. If you can get an audience together, do so. It will be endlessly amusing to them. Practicing transitions will do several things. Transitions--especially the first one when you come out of the water--can be chaotic and confusing. It helps to have a little muscle memory going on. It will also help you figure out where you want to put what, and in what order you're going to do things. Best of all it will just help you get a sense of what to expect.

Preview the course
If you can, head to the course and cover as much of the bike and run parts as possible. It will help you visualize the course before race day, and make it less intimidating. On race day, it will help you know what kind of effort you've got coming up ahead of you. Even if you can only do this in a car, it's still helpful. Be sure to get in the body of water you'll be swimming in, especially if it's an open water swim, ahead of time. Another helpful tip is to search for your event on MotionBased. This will help you gauge the elevation profile.

Put BodyGlide on the outside of your wetsuit.
On race morning, after you glide up your ankles, pits, neck, and other chaffing-prone areas, hit the outside of your suit around your forearms and ankles. Really smear it on there. It will help the suit come off quickly and cleanly.
Put a towel in your transition area under your gear
This does two things. It helps delineate the "this is my space" area, and it also gives you an easy way to dry your feet. You just step on the towel and walk around on it when you come out of the swim, and you're already drying off your toes. Remember: shoes on last.

Keep an extra squeezebottle full of water in your transition area.
You can use it to quickly take a drink, and spray off your face and feet when you come out of the swim (to get rid of the sand, dirt, mud, algae, and saltwater).
Attach your race belt to your running hat
This will save you an awkward step in your transition area of fumbling with your race belt. (And while we're at it, you should have a race belt with your number attached to it.) Loop your race belt through the opening in the back of your hat, and snap it closed. When you head out on the run, you can just put on your hat, and the belt will be there hanging behind you. You can then put it on while you're running, instead of in T2. If you have to wear your number during the bike, however, go ahead and snap it on in T1. You don't want to be fucking with a belt from a moving bike.
Bring a hard-boiled egg
Or a banana. Or both. It can be a long wait in between the time when you arrive in the morning to set up your transition areas and when your heat starts. Naturally, you ate a good breakfast that morning--even if it was at 3 am--with both protein and carbohydrate. But what happens when you get to the event and then have to wait three (or more) hours to start? You don't want to get hungry. You could just bring an extra bar or two, but I prefer real food. Bananas and eggs make quick, easy, clean snacks, and both give you great fuel.
Gear Check
The day before the race, put all your gear out together, including the warm-up clothes you're going to wear before and after the race. Use a checklist to make sure you have every single item, then pack it all away in a single bag. Here are a few examples of my OCD packing list. This one is very well annotated. Don't worry if you don't have everything I do; I have too much shit.
Laces Locks
You don't want to have to fuck with tying your shoes in your T2. Get some laces locks so you can easily get in and out of your kicks. They are awesome, and like $5.
Don't talk to motherfuckers before the race
People tend to get really, really nervous before races. Don't let that rub off on you. Stay in the zone, and don't talk to the chatty charlies around you. They're talkative because they're afraid. You are not afraid.
Don't do anything for the first time
This is the tip you'll most often hear, and it's true. On race day, don't use any new gear, eat any new food, try any different techniques, or anything else. You want to make sure everything you do you've done before (which means, if you're going to do any of the above, try it out at home first before race day).
You're going to be awesome
For real. You've been training for months. You've got your shit together. You'll come out on the other side a stronger, more confident person, with a finisher's medal to boot. It's going to be fine.
