Keep a detailed log of your library coworkers' anniversaries and other miscellaneous milestones. Don't limit your celebrations to library events ("Joe, congratulations on your fifth year in the Cataloging Department!"). Your coworkers will be flattered to learn that you keep a detailed record of their extracurricular activities, romantic relationships, and rehab milestones.
[Editor's note: Today marks the two year birthday of A Librarian's Guide to Etiquette. Celebrate by doing something rude to a coworker.]