Keep your body upright with the head out of the water. That way you stay in one place until rescue comes. If you need to move a bit, lean in that direction.
Hand and Arm Movements:
Make slow sculling movements underwater to maintain the vertical body position. Don't overexert yourself.
Make slow, continuous bicycle-like or scissor-kick type movements to support your body. Your feet move in a circle.
Raise your arms above your head and use just the legs to keep afloat. This is very exhausting.
Add more clothes as you get stronger. This is a great training exercise. Include it in each swimming session and team up with your friends.