Warm Up and Cool Down
Warm up for five to 10 minutes before your swim workout to prepare your muscles for exercise. Also be sure to cool down for the same amount of time at the end of your workout.

Boost intensity
While swimming, alternate between a fast and slow pace. This will allow you to relax a bit during the slow portions, so your faster swims can be strong and intense. Choose from the following interval routines—or do them all if you're advanced.

One-length intervals: Swim hard for one length, then easily for the next, and repeat four to five times, gradually increasing to 10 to 20 laps.

50-meter intervals: Swim hard for 50 meters (two standard pool lengths), then rest for 15 seconds. Repeat the interval and rest sequence four to five times. As you progress, increase the number of reps.

Deep-water running: Start in the deep end of the pool, treading water. Now kick with your legs as if you're running, without using your arms (keep them at your sides). Aim for one minute of kicking. Since this is a tough move, begin with 30 seconds as your goal before you progress. Repeat four to five times, or until you need your arms to help.

Build strength
Many pools have the following pieces of equipment, which help you increase the strength in your upper or lower body.

Hand paddles add resistance in the water, toning your upper body. Wear them for five minutes during your routine.

Kick while holding onto a kickboard for five minutes to tone and strengthen your legs. Equipment-free option: Try swimming with just one arm for one pool length (keep the other at your side), then switch arms. The one in use will have to propel your entire body, which increases strength and improves your overall stroke technique.

Quick Tip: Use different strokes to add variety—and work more muscle groups. For example, try the crawl for one lap, breast stroke for the next, backstroke for another.

Improve flexibility
After your workout, try these stretches to increase flexibility in your shoulders, back, and thighs. Stand in the shallow end of the pool, with the water waist level or higher, and hold each of the following moves for 20 to 30 seconds without bouncing.

Shoulders: Stand with your back to the side of the pool, arms straight behind you, grasping the pool edge with your palms down. Lean body forward until you feel a stretch in the front of your shoulders.

Shoulders and back: Now turn so the left side of your body is facing the pool edge, and grasp the pool gutter (or edge) with your left hand; your right hand should rest by your side. Lean your body away from the edge until you feel the stretch in your left shoulder and upper back. Hold, then switch arms, and repeat.

Lower-back stretch: Stand facing away from the pool edge. Look over your right shoulder as you twist torso to the right, and grasp the pool edge with your right hand. Repeat with other side.

Hamstrings: Go to the shallow end, and stand facing the steps. Bring your left heel onto a step (use whichever is comfortable), keeping leg straight but not locked. Bend torso over leg until you feel the stretch in the back of the leg. Hold, then switch sides.