I've never been asked whether I have any guns in the vehicle, but I wonder if responding with "I don't have anything illegal in the vehicle" would fly? You're sort of evading the question, and I'm sure some officers would catch it, but as long as you're within an exemption, you're not lying. Your next step would obviously be to politely decline any requests to search you or your vehicle.
The biggest thing IMO is to drive within the laws so you don't get pulled over in the first place. Then, don't give the officer probable cause, i.e. keep the inside of your car clean, and make sure your guns are cased (but not in "gun cases") or, ideally, in the trunk where s/he cannot see them.