One way your body may be getting bored with your cardio workouts is that you're always working the same muscles over and over again. In order to constantly be challenging yourself, it is a good idea to use different machines. OK, but what if you own an elliptical machine at home?
Don't worry, you can still switch things around (and still use the same machine) simply with how you use your feet while on the machine. Use your toes to really work your quads (front of your upper legs) a little more. Use your heels to really work your booty and hamstrings (back of your upper legs) a little more. I like to alternate two minutes of pushing with my toes with two minutes of pushing with my heels. You can also try this while biking and while doing stairclimbers.
Fit's Tip: I also find that this technique can help with toe numbness, which can be a big problem.