Know Where to Leave
With swimmers coming in, cyclists going out, cyclists coming in and runners going out (and sometimes, runners finishing at the nearby finish line), you'd be surprised how easy it is to get discombobulated when leaving a transition area.
While you're getting your area set up, make sure you make a mental note of where both the bike exit and run exit are located. Use mental imagery to envision yourself heading out of your transition area toward the right spot during both transitions.
Sometimes, the exits are on opposite sides of the transition area, so going to the wrong one could add up to a minute to your total time.
If all else fails, ask a volunteer; there should be one close by. Or, follow the other triathletes and hope they're right.
Practice Running With Your Bike
Typically, you won't be allowed to get on your bike until you're out of the transition area. To best get out of the area, you will need to run with your bike.
The fundamental way to do this is to run alongside your bike with one hand on the seat or handlebars and the other hand free. Then, when you're allowed to mount, get on while you're still moving.
The best way to get good at this is to go out and practice. Go to a school parking lot on a weekend and work on it—both running with the bike and mounting on the run. Get comfortable—not only will it boost your confidence come race day, but it will cut a few seconds off your final time.
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