A person cannot prevent the natural shrinkage of cotton fibers, according to About.com. The best thing to do is to buy "pre-shunk" cotton clothing to deter shrinkage after purchase. For existing clothes, a few steps mitigate shrinkage.
Read the label
Read the label for special care instructions. A particular garment may have specific requirements.
Wash in cold water
Wash 100 percent cotton clothes in cold water. About.com indicates that the delicate cycle and slow spin are the best settings to use because they prevent cotton fibers from breaking easily.
Hang cotton clothes on a clothes line to air-dry. The artificial heating process in a clothes dryer contributes to shrinkage.