As many have already said, the most straightforward answer you can give is also the simplest, the truth. By the time your car reaches the booth, the decision to refer you to secondary is likely already made. If not, lying about something or seeming overly flustered or nervous is sure to increase your odds of referral. Having crossed the border in multiple places, and in both directions, for the past 35 years, there are some patterns that I have noticed which increase questions asked and the odds of referral: driving with non-family (especially other people's children), driving in a vehicle you don't own (not rental), showing up at a land crossing when you live 2,000 miles away, towing a trailer (of any size), being between 18-45 years old, having a thick accent (of any kind), not comprehending the questions being asked, giving answers to questions that weren't asked, being overly friendly with the agent, being overly aggressive with the agent, telling dumb jokes about bombs/terrorism/drugs/illegal immigration etc. And of course, there's the luck of the draw when you randomly run into the a**hole of the month when he's itching to make someone's life just a little more difficult.
All in all, it's not the most pleasant experience you'll ever have but CBP and CBSA agents are mostly harmless.