When you are trying to build strength and build muscles, we want to attack as many muscle fibers as possible, explains sports conditioning coach Fiona Lockhart. That means upping the weight and decreasing the reps. Fifty biceps curls might build muscular endurance but you're not going to build the strength you're looking for, Lockhart says. Of course, it also takes a lot more time to do 50 reps with light weights than 10 to 15 reps with more weight. If you're able to do more than 15 repetitions of an exercise, it's time to increase the weight, Lockhart says.

The same is true of cardiovascular exercise. It's easy to hop on the treadmill and type in the same speed, incline, and time every single time. But your body gets used to it. If you're trying to maximize time at the gym, work at a higher intensity for a shorter time, says Teri Trese, MS, a fitness trainer at Pritikin Longevity Center & Spa. If you can get and stay near 85% of your target heart rate, you'll accomplish more for your total fitness.

You get to the weight room and float around until you find an open machine. Then your time is over, and you've only gotten through three or four exercises. Think about what you're going to do in advance, then stick with it, says Lockhart. If it's cardio, then get on the treadmill or bike and focus. Throw in some two-minute intervals.

When you work out with a friend or friends, set some rules first to be sure everyone stays on track with time, Trese advises. Try doing 8 to 10 exercises in 30 minutes, and resting no longer than a minute between exercises. You can also save time during your warm-up by mimicking exercises you'll be doing in the workout. For example, Comana says, if you plan to work your legs by doing lunges and squats with weights, warm up with high knee steps, butt kicks, lunges with a twist, and sumo squats. For more details visit http://www.soundbodytrainer.com/