Hills are our friends. They might not seem like it as you are struggling to run, walk or ride up one, but they are a great way to add intensity and extra calorie burn to your workout. At least once a week find a route with some good hills or when you are on the treadmill bring up the incline. If you're on an indoor bike or spin class, turn up the tension. Repeat the hills a few times, trying to get a little faster each time. You will see a difference in your overall cardiovascular strength and fitness level.
Link to blog: http://thefitdivas.typepad.com/my_weblog/2009/03/hills-hills-hills.html