1. Always add 's.
2. Add 's to words of one syllable; add just an apostrophe to words of two or more syllables.
3. Add 's only if you pronounce it as an additional syllable.
4. Add 's unless the additional syllable would result in a repetitious sound (as with Jesus, Moses, Xerxes, etc.)
I was taught #2, but in the past few decades, most style guides I have consulted have switched to #1. This is probably the clearest method.