Burn 300 Calories in 30 Minutes

          If you're trying to burn calories and lose weight using cardio machines, I have a challenge for you: Put down the magazine, turn off the television and stop looking at your watch.  Okay, you can keep the iPod or MP3 player, but everything else has to go.  Why the drastic measures? If you aren't paying attention, you may not be working as hard as you think.
Cardio Workouts to Burn More Calories
If you're used to zoning out at the gym, he following workouts will help you pay attention to what you're doing, change the settings on your machine and burn more calories. Modify each workout according to your fitness level and feel free to increase or decrease speed, incline and/or workout time to fit your needs.
For each exercise, warm up with 5-10 minutes of light cardio and keep track of your perceived exertoin  or monitor your heart rate. Use this calories calculator to determine how many calories you're burning. End with a cool down and stretch.
Treadmill

• Start with incline at zero and speed at a comfortable pace (walking or jogging). Perceived Exertion (PE)=Level 5
• 1 Minute: Raise incline one or more percent every 15 seconds. PE=5-6
• 1 Minute: Reduce the incline one percent every fifteen seconds. PE=6-7
• 3 Minutes: Walk or jog at a steady pace. PE=5
• Repeat entire cycle for 30 or more minutes
• Approximate calories burned: 320 (based on 140-lb person)

Elliptical Trainer

• Using manual program, enter workout time as 30 minutes and choose appropriate level
• 6 Minutes: Set ramps (if option) and resistance levels at medium level. PE=5
• 2 Minutes: Set ramps at highest level and increase resistance every 30 seconds. PE=6-8
• If you don't have ramps, set your resistance at the highest level you can manage for 30 seconds
• 2 Minutes: Lower ramps and resistance to comfortable level. PE=5
• If you don't have ramps, lower your resistance until you're at a comfortable level.
• 6 minutes: Set ramps/resistance to medium and go backwards. PE=5-6
• Repeat entire cycle for remaining time
• Calories Burned: 250-300 (based on 140-lb person)  
  Bike
• Using the manual program, enter your workout time as 30 minutes and choose appropriate level
• 5 Minutes: Cycle at a comfortable pace. PE=5.
• 2 Minutes: Raise your resistance 2-3 increments every 30 seconds. PE=6-8
• 2 Minutes: Lower your resistance 2-3 increments every 30 seconds. PE=6-8.
• 1 Minute: Set resistance at a moderate/high level and cycle as fast as you can. PE=8-9
• Repeat the entire cycle for 30 or more minutes
• Calories Burned: 245 (based on 140 lb person)

Hit the Road
Instead of spending 40 minutes inside a sweaty gym, head outside for a walk/run.

• Warm up with a brisk walk/slow jog for 5-10 minutes.
• Jog/walk briskly for 3 minutes
• Sprint or speed walk as fast as you can for 30 seconds
• Repeat this cycle for 20-30 minutes, keeping PE between 5-9.

As you can see, it's easy to spice up your workouts by playing around with your speed, incline and resistance and alternating high intensity intervals with recovery periods. This not only keeps you from getting bored, it also helps burn more calories and increases your endurance very quickly. Use this technique with any cardio activity to keep things interesting.