From my (limited) experience:
1. Make sure you know the routes; where the swim exits and the route to/from transition. What route does the ride and run take? Are there any danger points on the ride you need to be aware of?
2. Pace yourself. It's no good flying through the swim and the bike to run out of steam on the run

3. Use a tri belt rather than pinned numbers twice. That way you can carry some gels and just turn the belt round. Elastic laces are also good. Unless they snap!
4. Organise your kit (L to R if you're right-handed). Many have clear plastic boxes to make this easier - I use a black bag but it does the same thing.
5. Train for the distances and beyond. But don't burn out.
What tri are you looking at out of interest?