David Ogilvy says many sage things in his iconic “Confessions of An Advertising Man”, but probably my favorite passage is this, talking about how to choose a firm to handle your marketing:

Find out if they are discreet about the secrets of their present clients. Find out if they have the spine to disagree when you say something stupid. Observe their relationship with each other; are they professional colleagues or quarrelsome politicians? Do they promise you results which are obviously exaggerated? Do they sound like extinct volcanoes, or are they alive? Are they good listeners? Are they intellectually honest?
Above all, find out if you like them; the relationship between client and agency has to be an intimate one, and it can be hell if the personal chemistry is sour.

I couldn’t have said it better myself.