1. Get a good night's sleep tonight. The night before the meet, your sleep doesn't matter so much; the night before the night before, it does.
2. Get a good warmup, including some practice starts, before your event.
3. In the last 80-120m of the race, if your legs feel tired or heavy, switch your brain to your arms. But don't just think "arms"--concentrate on the backward swing of your elbows (the only part of the armswing that actually pushes you forward).
4. And regarding those elbows: they should be relaxed the whole way. That doesn't mean your arms will be flopping around! It means that, when your arm swings back, the weight of the forearm will open the elbow some; as the arm swings forward, the weight of the forearm will close the elbow some. This relaxed open-and-close is actually very natural and most sprinters do it well, but sometimes in the last part of a 400 the runner locks up the elbow.
5. In the homestraight you can combine tips 3 and 4 by thinking "relaxed-back-relaxed-back."
Good luck. Let us know how it turns out!
By the way, here's a picture of someone sprinting with the elbow opening some (as it moves behind the torso) and more closed (when it's in front of the torso):
http://blogs.reuters.com/photographers-blog/files/2013/09/RTX13NYL.jpg