If mention of degrees is necessary to establish someone's credentials, the preferred form is to avoid an abbreviation and use instead a phrase such as: John Jones, who has a doctorate in psychology.
Use an apostrophe in bachelor's degree, a master's, etc., but there is no possessive in Bachelor of Arts or Master of Science.
Also: an associate degree (no possessive).
Use such abbreviations as B.A., M.A., LL.D. and Ph.D. only when the need to identify many individuals by degree on first reference would make the preferred form cumbersome. Use these abbreviations only after a full name -- never after just a last name.
When used after a name, an academic abbreviation is set off by commas: John Snow, Ph.D., spoke.
Do not precede a name with a courtesy title for an academic degree and follow it with the abbreviation for the degree in the same reference:
Wrong: Dr. Pam Jones, Ph.D.
Right: Dr. Pam Jones, a chemist.