[Tdol]

It's not an exact science, but here are some guidlines from a style book that seem reasonable:

Words already hyphenated should be broken at the hyphen.

Break words according to their derivation: aristo-cracy, melli-fluous.

Words of one syllable should not be broken.

If possible, at least three characters should be taken over to the next line.

Words should not be broken so that their identity is confused or their identifying sound distorted (e.g. rap-ist).

Personal names should not be broken.

Figures should not be broken or separated from their unit of measurement.

A word formed with a prefix or suffix should be broken at that point.