What are effective guidelines for using [interval training](http://en.wikipedia.org/wiki/Interval_training) to improve distance running performance and/or maintain physical fitness? 

My high school cross-country coach used to have the team run intervals once a week. Are there any guidelines or research into how often intervals should be mixed in with a runner's regular regimen to either maximize performance/endurance or reduce injury risk? I've seen many sites that discuss the benefits of this form of training, but fewer that discuss a "standard" regimen that can be used for a wide range of runners.