What's the difference between farther and further?

Farther means "more distant" and is usually measurable, as in these examples:

Is El Paso farther from Dallas or Los Angeles?
John decided to climb farther into the cave.

Sometimes farther can be used more figuratively, such as:

My best friend and I drifted farther apart.

Further is a more abstract term meaning time or quantity ("more" or "additional"), as in these examples:

Katie developed further complications.
Jim needs to research the subject further.

In writing, you'd be okay to use further any time you don't apply it to a distance.