This is really Jay J's area of expertise but he is off for a long weekend so I will try to cover for him.
Sean, get a copy of the current The Family Handyman magazine. The major booksellers and some home centers have it. There is a nice article on dealing with water run-off. But to try to answer your specific question, you surround your french drain with the long lasting landscape fabric. This filters out the dirt particles from the water. If you are running perforated pipe, the fabric is available in long tubes called socks that go over the pipe to do the same thing.