What is the "right" answer to give when a law enforcement officer asks "Do you mind if I search your vehicle?"

We've all seen "Cops" and/or heard stories from friends on BOTH sides of the equation - cops who tell you of pulling over little old ladies for a busted taillight only to find $500,000 worth of crack in the trunk - or people who literally were breaking no laws, but they were "randomly" pulled over, and "acted nervous" so the police decided to search their car, found a crack pipe their teenage daughter's friend dropped in the back seat, and the next thing you know their life is ruined. What is the right answer to that question "Do you mind if I look in the trunk?" How do you answer that and preserve your rights? And refusing the request - isn't that probable cause for the officer to ume you have something to hide - meaning he/she is going to proceed with the search anyway? If there are any legal eagles or law enforcement officers out there who would like to posit an answer, I would really appreciate it. I think this is a fascinating issue, and I'd love to see the facts and opinions.