Swimmers use hypoxic, or oxygen-reduced, training to increase their lung capacity, and to improve their tolerance for oxygen deprivation. Hypoxic training basically involves swimming different drills while reducing the amount of air a swimmer breathes. The effectiveness of hypoxic training isn't settled science, but oxygen-reduced training drills are a staple of many competitive swimmers' daily workouts.
Swim a 25-yard freestyle lap without taking a breath. Hold your breath for the first few strokes of the lap, then begin to exhale very slowly as you complete the distance. Exhaling rapidly keeps your body from using all of the oxygen in your lungs and causes your muscles to build up oxygen debt, which will hinder the rest of your workout. Rest for 30 seconds, then swim another lap without breathing. Swim 10 reps in your first workout, then add a couple of laps each day until you can swim 20 no-breathers.
Vary your breathing pattern while swimming freestyle 25s. Begin by adding one or two strokes to your normal breathing cycle. Swim a set of 10 breath-control laps in your first few workouts. Rest for 30 seconds between each lap, and don't overdo it; shorten your breathing interval if the rest interval doesn't allow you to completely recover. As you build tolerance for the lactic acid buildup in your lungs -- that slight burning sensation you feel in your chest -- add more strokes to your breathing cycle until you can comfortably breathe once every eight or 10 strokes. You can also focus more on extending your breathing pattern without changing your stroke by swimming freestyle laps with a front snorkel.
Swim a full 25-yard lap while staying completely underwater. Try three or four of these to start with, and rest for 60 seconds between each lap. Work your way up to seven or eight underwater laps each day. If you can't complete an entire lap underwater, see how far you can go before you need to surface. Younger swimmers usually find it easier to swim underwater using a breaststroke pull-down. More experienced swimmers can try their underwater laps by swimming in the streamline position and using their dolphin kick.
If you're struggling to complete no-breathers or breath control drills, try minimizing your kick. Your legs and hips contribute substantially to your speed, but they also are the largest muscles in your body and consume the most oxygen. It's okay to swim a bit slower in breathing drills if it reduces your body's oxygen demand.