If you're spending a lot of time on one specific type of cardio machine, you may want to add in some variety. Using the same machine can lead to overuse injuries because you're always using the same joints and muscle groups.

One obvious way to vary your routine is to use a different machine during each gym visit. Or, if you really want to spice things up, you could switch machines halfway through your workout.

For example, instead of 30 straight minutes on the treadmill, do 15 minutes and then go to a step climber for the next 15 minutes. Other "cardio combos" could include rowing machines, elliptical trainers, or stationary bikes.

In switching from one machine to another, you'll want to make sure you continue to work at the same intensity (or better). Use the machines' pulse monitors as a guide. And, of course, make your switch right away; stopping to talk to a friend or to take a breather will make your workout a little less effective.