Training using intervals is the absolute best way to burn calories during and, for hours, after your workout.  The traditional method of working at the same pace or intensity for your entire workout is old school.  Gone are the days of jogging a 9 minute mile for 30 minutes.  Today we know that changing up your heartrate while you exercise is the best way to get in shape.  Even with only 15 to 20 minutes to workout, you can burn hundreds of calories.  Walk briskly for 1 minute, run for 1 minute; jog for 2 minutes, sprint for 30 seconds; perform 2 minutes of high intensity cardio between lifting sets, bike at a Level 2 resistance for 3 minutes, then at a Level 8 resistance for 2 minutes.  Your interval training options are endless.  Change it up each time you workout to keep your body guessing and maximize the amount of calories you burn, and increase the heart benefits of your exercise program.