The most important element is chemistry. Public relations is a judgment skills business and it is imperative to ascertain the values, ability and judgment of the principal account executive assigned to your account. Compare the personalities of your firm to the agency. Request to see a list of active clients with telephone numbers and contacts and talk with the clients. Ask how they worked together and would they hire them again.
Check accounts lists for conflicts.
Compare size and prestige. Visit their offices. Carefully consider whether an agency's expert in New York will be worthwhile after you pay travel and conference call expenses. See if the agency volunteers how they saved clients money. Look for evidence of work product.