The Japanese professor Izumi Tabata studied training techniques with Olympic skaters and developed a routine that bears his name today. They are called tabata's and they consist of training at a high intensity for 20 seconds, resting for only 10 seconds, and then repeating the high intensity interval 7 more times. These 8 repetitions at 20 seconds on and 10 seconds off, racked up a work-out totaling 4 minutes! Tabata trained his athletes four times a week this way and noticed conditioning gains similar to athletes that trained longer and more often.
   A simple way to do a tabata is to move your body vigorously in any way (run, jumping jacks, push-ups, squats, spinnning, etc) for 20 seconds, rest for 10, and repeat with the same or another exercise. Do 8 reps and you are done. Do this at least twice a week, preferably 3 times, and you can improve your conditioning without spending hours in a gym.