Going gray is a natural part of the aging process. It is caused by a decrease in the production of melanin, the pigment that gives hair its color. While there is no way to completely stop going gray, there are a number of things you can do to slow down the process.
One of the most important things you can do is to protect your hair from the sun. UV rays can damage hair and cause it to lose its color. Be sure to wear a hat or scarf when you are outdoors, and use a leave-in conditioner with UV protection.