It would seem natural that your heart rate monitor should be your greatest ally when it comes to race day but as ever, the reasoning is not that simple. While you can use your experience from previous competitions to your advantage, you'll find that you can't just transfer what you've learnt from ordinary training onto the race course.
If you've worn your HRM in races before, one of the first things you'll have noticed is that, unlike in training, your heart rate rises extremely quickly once the race starts. You'll also have seen that the readings are much higher than usual.
Research from South Africa has found the same thing. The conclusion is that it just wouldn't make sense to base your racing heart rate on heart rate figures from non-competitive training sessions. Racing is clearly different from ordinary fast running, and it's largely because in a race, your heart rate is affected by more than just the amount of work your body is doing. One of the major influences is thought to be arousal and the associated adrenaline surge that accompanies a competitive situation, though this has yet to be scientifically established.
If you did use your HRM in a race and paced yourself according to your training rates, you'd probably be holding yourself back rather than gaining an edge. For a start, the anticipation and adrenaline we've mentioned will give you artificially high heart rate readings, which won't reflect your pace. In addition, you'll often find that in a race situation you can maintain a higher heart rate than you're used to anyway.
Heart rate information from training sessions can only occasionally be useful in a race situation. You may have decided to use a race simply as a controlled training run over an accurately measured distance, for example. Alternatively, you could be tackling a new distance with the sole aim of maintaining a steady 'get-you-round' pace. Otherwise elementary as it may sound research shows that the best approach to a proper race is to simply aim for a fast but level pace throughout.
The only useful way to establish a heart rate for racing is to do it retrospectively using real race data. Ideally, you need to be able to store your rates on your monitor as you run and then plot them on a graph afterwards. At the beginning of the graph, you would expect to see a sharp increase in your heart rate. Then, if you've raced well, the trace will remain high but virtually constant. It may increase by a few beats, but the only real variation should be a possible small jump at the end as a result of a finishing kick.
After a few races, you'll be able to identify your normal racing heart rate (though remember that factors such as heat and hilliness will have a pronounced effect), and then if you have a bad race, the graph can give hints as to why. One common cause is infection, which normally causes the heart rate to be much higher than usual throughout, despite a lower running speed. Another reason might be that you've simply started too fast. In this case, you struggle (unsuccessfully) to maintain your pace, but despite the fact that you're still working hard, your heart rate drops throughout the race. This pattern is not uncommon and can also be seen in overtrained athletes, who can find it hard to maintain their normal high heart rate despite putting in a lot of effort.
Whatever your performance, a decent collection of heart rate graphs will help you pinpoint the factors that made your race a success or otherwise. Then it's just a question of looking to the future.