This tip should really be something that goes without saying, but a lot of people actually don't do this! If there is any possible way, I highly recommend that you go out and preview whatever race course you will be racing. Whether it's a swim, bike, run, all of the above, or whatever, there is a ton of information you can gain from getting out there and doing the course prior to race day.
For swimming, if you can get in the water and figure out some large objects that are easy to see to use for sighting, instead of the buoys that are typically hard to see. You also get a feel for the water and whether or not it's choppy, cold, dirty, clean, whatever.
Biking on the course will allow you to learn first-hand when you can push and when to relax. There may be times that seem flat to your eyes, and yet are actually a gradual incline. Or maybe there's a hill with a curve on it, so you can figure out how long the actual climb is. You also get to learn the condition of the road and if there's any large potholes or dangerous spots to look out for.
Running early has similar benefits to biking. Another added bonus is that you can figure out which way the course actually goes, so if you find yourself alone at some point in the race, and by chance the course is poorly marked, you still have a general idea of where to go!
There are a ton of other benefits to getting out on the race course prior to race day, and I'm sure you can probably think of some that I didn't mention. The point is, there are a lot of benefits. It's that simple! So next time you have a race coming up, and assuming it's possible to do so, go practice your race on the course!