Interval training has been around for years, is tried and tested and highly regarded as a training method that really does increase your speed. The old ways are often the best...
By working in short bursts with recovery periods between intervals a higher intensity workout can be achieved, increasing cardiovacular fitness and muscle strength.
It's advised that you build up interval training gradually and don't incorporate more than one or two sessions into your weekly schedule to avoid overtraining. In one study people were made to perform intervals every day, and within two weeks their performance was actually slower as their bodies were not given time to rest and recover from the intense training.
It can seem pretty boring, but you don't have to pound up and down the same bit of pavement - choose a small area you really like to run in. Find a hill for an extra challenge, take the iPod, listen to the radio, whatever motivates you. If it's pouring with rain you can easily adapt intervals to the treadmill. An interval session is likely to be a shorter training session than you're used to, so you might be able to fit it in at a different time of day or on a day when you wouldn't normally have time to run. Read this article for ideas on how to include intervals for different types of exercise.
Remember, quality not quantity - a 20 minute interval session (with warm-up and cool-down) may well do more to improve your speed than a 50 minute slow and steady run.
Have a look at the video below explaining how and why interval training works.