This is from the Strunk and White's Elements of Style, Fourth Edition.
1. Form the possessive singular of nouns by adding 's.
Follow this rule whatever the final consonant. Thus write:
Charles's friend
Burns's poems
the witch's malice
Exceptions are the possessives of ancient proper names ending in --es and --is, the possessive Jesus' and such forms as for conscience' sake, for righteousness' sake. But such forms as Moses' Laws, Isis' temple are commonly replaced by:
the laws of Moses
the temple of Isis