From Sharon Norris Elliot: I use the words in different situations. "Farther" deals with distance traveled; i.e. I can drive farther than you can drive. Further deals with continuing something like a talk; i.e. I will speak further on the matter later.
Farther is a continuation of "far"; further is a full length fur.
From Jane Rumph: Here is the traditional rule in formal English: If you can measure the distance physically, no matter how big or small, something is "farther." Otherwise--and this includes abstract relations of degree or extent--the word you want is "further."
"With my bad foot, I could not walk farther than the corner."
"Jupiter is farther from the Earth than Mars."
"Farther north, the snow begins in September."
"I read farther down the page and found what I was looking for." (measuring space on a page)
"I read farther into the book for more details." (measuring pages in a book)
BUT: "I read further about that event." (your reading continued; no physical context specified)
"I wouldn't go any further on that topic."
"Nothing could be further from the truth."
"The project developed further once John came on board."
"Romeo couldn't get any further than second base with her." (The metaphor of second base does imply physical distance, but since this sentence is not talking about a baseball diamond, I would still use "further.")
Some style manuals say this distinction is not consistently maintained, and that either word can be used to indicate "distance to go," while only "further" should be used to mean "more." But physical distance is a good rule of thumb.