As Jodi the Trainer pointed out in her latest blog, "The Importance of Varying Exercise Intensity," there are a couple different approaches to cardiovascular training, and Interval Training is quickly becoming the most popular.
During Interval Training, you should be working at or above 85% of your max heart rate for a period of time, followed by a recovery period at 60-70%. This form of training has a lingering effect on your calorie burn - meaning you can burn up to 30% more calories after your workout than you would normally burn throughout the day.
As Jodi suggested, stick to interval training 1-3x/week, or 50% of your overall cardio training.
To help you get started, we have put together a 20 minute calorie blasting interval workout for the treadmill!