If you are ever alone in the water for possibly an extended period of time, you can inflate your clothes into Personal Flotation Devices (PFD), which can help you conserve energy and stay afloat for much longer than you could by treading water alone - potentially saving your life.  Using your clothes as a flotation device is also known as drownproofing.

Inflating Your Pants as a PFD

1. Start by treading water for a few moments until you calm down and can logically begin to get into action. A panicked person is not going to be able to make clothes into flotation devices but they may drown trying. Continue to tread water throughout the entire process.
2. Kick off your shoes. Your shoes are not going to help you float and will only serve to weigh you down. They will also hamper removing your pants, which is the first item you are going to fashion into a flotation device.
3. Squirm out of your pants and adapt them to float. Once your pants are off, get them in front of you while you continue to kick your legs to stay afloat. Quickly tie the end the legs into a knot and close the waist and zipper.
4. Still treading water, squeeze as much water as you can from the pants and then wave them over your head to fill them with air.
5. Once air is inside, close off the waist area underwater the best you can to keep it trapped and floating.
6. If you need more air, while keeping the waist underwater, dip your head underwater and blow air into the waist.

Inflating Your Shirt as a PFD

1. Develop a strong consistent kick for treading water.
2. Pull collar up on back of neck.
3. Pull front of shirt up over mouth. (Keep nose out of shirt)
4. Fold / bunch / pinch excess shirt around mouth.  This is important, without a good seal air will leak out around your neck.
5. Inhale through nose, exhale through mouth, into shirt.
6. Lean forward, to send air to the shoulder area.
7. Continue inflating as necessary, to stay afloat.

Sources:

• https://www.youtube.com/watch?v=vAhUD6_gS6w
• http://www.ehow.com/how_2338250_make-clothes-flotation-devices.html
• http://freelifesavingsociety.com/swimming/Survival/drownproofing/inflate-clothes.html
• http://www.hawaiiarmyweekly.com/2011/04/01/extreme-challenge-jrotc-cadets-climb-swim-rappel-in-islandwide-leadership-challenge/