Every rights/freedom activist's line in the sand should always be, "I do not consent to a search".
But just for grins, and to put things in perspective for the courts (should it come to that), I suggest that the first response to "You don't mind if I search your car, do you?", be along the lines of "Why in the world would you want to search li'l ol' me? What do you think you might I might have?"
That forces the officer to articulate what he's searching for; with that statement, he limits where he can search. If he's searching for guns (for his own safety, of course), the ashtray (and the roach within) is off limits. As would be the folded paper bag of cocaine in Florida v. Jimeno
; there are no guns that can be concealed in a gum wrapper.
Courts have long held that the authority to search is limited to the place and time where the person or thing to be seized might reasonably be found. A warrant for a stolen elephant doesn't authorize police to look in your medicine cabinet; a search for an escaped convict doesn't permit them to look in your glove compartment; a Terry pat-down for officer safety doesn't give the officer leave to dig through your pockets.
"I don't know what you might hope to find, Officer. What are you looking for?" is a good question. "Contraband" or "drugs" or "weapons" or "you never know what we might find" are all legally unacceptable answers. They show a lack of specific articulable suspicion about the likelihood of crime being "afoot", and instead illustrate that it's just a fishing expedition.