1. Rev it
Keep your heart rate at around 80% of your max for as much of your workout as you can. (To estimate your target rate, subtract your age from 220 and multiply by 0.8.) To ensure you're staying in the zone, stop after every 10 to 15 laps and use the pool clock to count your pulse for six seconds; tack a zero onto the number. Vigorous swimming can torch up to 680 calories per hour, based on a 150-pound woman.
2. Play around
Swim with a toy, such as a kickboard, hand paddles, swim fins, or a foam buoy that fits between your legs. Most pools have them, or you can buy your own at a local sporting-goods store. Not only will you burn more calories, but you'll tone arm and leg muscles, too.
3. Mix it up
Break up your laps into what pros call a ladder. Swim the following segments with 15 to 30 seconds of rest in between each: one lap, two laps, four laps, six laps, four laps, two laps, one lap.
Nothing burns calories better than swimming fast. But you don't have to speed through your whole workout to get the benefit. Try doing one length easy, then one length fast; two lengths easy, two lengths fast; and so on. Or divide your workout into four to six segments and swim one fast lap at the end of each.
5. Rest less
No more than 10% of your water time should be spent loitering in your lane. Try cutting your rests in half, until you're pausing just 10 seconds between laps or intervals.