"I'm sorry Mr./Mrs/Sargent/Lt X, but I don't talk to police on material matters without counsel present."
I grew up around law enforcement (best friend's father was FBI, and current friends in Secret Service etc.). Also, in Fed-land (e.g. Northern Virginia) where nearly everyone's family is in some form of government/legal service, I suspect the public/police interactions are atypical.
As a result, talking to cops in both personal and professional contexts isn't a novel experience. IMO, Virginia also has professional cop culture.
I always recommend folks view this lecture given by Virginia's Regent Law School ( http://www.youtube.com/watch?v=i8z7NC5sgik ).
By immediately asserting your rights, the officer in question knows that they are not dealing with a person of "legal ignorance". As with communication in all social interactions, an assertive (but not aggressive) carriage, posture, positive eye contact, and tone helps a great deal.
Keep in mind that, in most contexts, when I see the triplet of a badge, an open-carry gun (the gun is key), and a police uniform, my default assumption is that of a PERSON who's sworn an oath and, consciously or unconsciously, respects SOME sort of legal authority given their avocation (though it may not necessarily be true).
FWIW, if you have any doubt, it's worth asking CLEARLY if the presumed officer in question has "sworn arrest powers in <insert municipality>". Police are allowed to lie, but they're not legally allowed to "assert police powers" such as arrest and/or detainment unless they have truly have them. In most U.S. state codes, there's a clear delineation between what constitutes a "lawful" and an "unlawful order" by law enforcement. Asserting police powers without being a sworn law enforcement officer is also usually a felony.
At worst, you may be severely inconvenienced for an "officer's safety" by being handcuffed and/or temporarily detained on-site, but you've preserved all of your rights under the law. Fighting temporary detainment is a "sticky wicket" and generally ill-advised unless you genuinely feel that your life or the lives of your family are at immediate risk (e.g. driving your pregnant wife to the hospital, etc.).