OK, so you've been doing cardio about five days a week and you're feeling great, but the results have kind of diminished. One way to pump things back into gear is to have one or two days where you really push yourself. This is what I like to call a 'workout of champions.'
Basically what this means is to challenge yourself in your cardio efforts just one or two days a week. Some ideas include sprinting on the treadmill (or outside) or taking that super tough fitness class that you've shied away from in the past. You may be totally wiped out afterwards, but you'll feel great knowing that you've pushed yourself.