Due diligence and personal service

CPLR 308(1) and (2) requires personal service unless it cannot be accomplished with due diligence, in which case "nail and mail" service is sufficient, pursuant to CPLR 308(4). The question for the solo practitioner, of course, is what exactly constitutes due diligence in the matter of personal service? In Akler v. Chisena, decided on May 1, 2007, the Second Department found the requisite due diligence -- and allowed nail and mail -- when plaintiff's process server had tried three times to serve defendant at his residence, but to no avail.