We all get at least some gray hair sooner or later -- it's in our genes, and there's not a thing we can do to prevent it. However, there are ways you can tone it down, blend it in or cover it up if totally natural is a bit too natural for you.
For complete coverage of hair that's more than 50 percent gray (or if it grows in patches) you'll need to use a permanent process. But, unless your gray is very premature, resist the temptation to go darker than your natural color -- it will just look harsh. Instead, pick a shade close to your natural color -- your roots will be less obvious, and your friends will still recognize you.
If your hair is less than 50 percent gray but still fairly noticeable, you can probably use a strong semipermanent process. If you use either of these products yourself, keep in mind that gray and white hair tends to be resistant, so you might have to leave the dye on for the maximum amount of time. Do a strand test first to get it right.
To blend in just a little gray, gentle semipermanents and temporary colors are good choices and are easy to apply at home (just beware of buildup); the gray will turn to highlights, and you'll look better than ever. Temporaries can also be very useful if you've decided to let nature take its course -- but still need a little help. They can take the yellowish cast out of white and gray and increase the "pepper" component of salt-and-pepper hair.