Almost any exercise is good exercise.
That said, some people waste time on routines that aren't best for reaching their goals, trainers say.
Here are general tips from Shaun Knight, a certified athletic trainer in Virginia Beach, Va.:
* Think about variety, not just time. Keeping your body guessing is the best way to torch calories. "If you ride an exercise bike at the same level for an hour, it may not be beneficial -- especially given the time you're putting in," Knight says. "Your body will get used to it and start to think it's easy."
* Add different elements. Instead of running at a steady pace for an hour, you might run for 20 minutes, bike for 20 minutes and run stairs for another 20. For strength-training, you could rotate between push-ups, squats and rowing machines.
* Switch up intensity. Increase your speed or resistance for short bursts of time. Even if you're just walking on a treadmill, you can make the incline steeper.
* Focus on more than one muscle group. Your abdominal muscles won't look great from crunches alone -- you need a full-body aerobic workout, too. You also won't effectively tone your legs just by using sit-down resistance equipment; instead, try a standing "cable column" machine that requires balance and works muscles from different angles.
* Schedule time with a trainer. Even if you can afford just one session, you can learn correct form for favorite exercises to prevent wasted energy and injury.