If you want to run faster on race day, there comes a point where you have to run faster in training. Long, steady runs are fine for improving your base endurance and threshold runs are great for boosting your aerobic efficiency, but to cap this with some speed, you need to turn to interval training.
In an ideal world, you'd be able to use your HRM for every kind of training, but it's less than perfect for short intervals. Imagine running 8 x 1-minute reps with three-minute recoveries. Unfortunately, because it's hard to raise your heart rate early on in a session using reps of just a minute, your readings would be quite low to start with. As a result, aiming to reach, say, 170 for each rep, you would probably start working too hard too early and start to deteriorate halfway through the session, despite long recoveries. You would have actually been better off without an HRM to distract you in this case, and this is where many eager runners go wrong.
Even if you take a steadier approach to a session made up of short reps, you still have to bear in mind that it takes a minute at the very least for your heart rate to reach a steady state and reflect the intensity at which you're working. This means that your heart rate monitor starts to give meaningful advice just as you finish each rep not terribly useful for controlling your session.
There are two solutions. If you're doing shorter reps, then you're better off running them to a set distance rather than a set time, and regulating your effort using a stopwatch. If you're using your HRM, then do longer reps with shorter recoveries (say, three minutes working and one minute resting), and start to take readings from the second rep. A good alternative to having a set recovery period is to rest until your heart rate has dropped to a certain level (you'll have to use trial and error to find this level). Though your recovery times will probably increase through the session, it should ensure that the quality of your efforts remains high.
Even on longer reps, you'll still need to use trial and error to find a pace you can sustain over the whole session before you can start using your HRM. Once you identify the heart rate corresponding to that pace, you can use the alarm on your monitor to tell you if you're pushing too hard. This is especially useful if you're performing quite a number of repetitions. At this point, you should take on board a word of caution: don't rush to reach a target heart rate in your intervals as we've said, it will take at least a minute for your heart rate to stabilise. The important thing to remember is that your HRM is there mainly as a passive observer. Its best function in interval training is to collect data for comparison over time, and throwing caution to the wind by immediately aiming for a set heart rate is a sure-fire way to ruin a track session.
Plotting your heart rates over a number of weeks should give encouraging results. Comparing two sessions a few weeks apart, for example, should show a slightly lower heart rate during the reps and a better recovery rate in the later session and you might be able to add on an extra repetition. All of these will be sure indicators that your training is going in the right direction.