Wow! Such an educated, whise advice! Everybody know these employees are control freaks! Nobody could ever think they're actually just doing their jobs and, after seeing a few dozen jerks by shift, maybe their sense of humor and manners might be affected so, it's totally normal to act as jerks with them! NOT!!!
The way to go? Be honest, don't play dumb, don't invent stories and, most important, don't play "smart ass" because 9 out of 10, they'll win over you. If you're telling the truth, chances are you won't get confused and won't give them bad answers and change your mind if asked again.
These peoples see all kind of peoples, from the worst crook to the most innocent old lady. They can possibly miss the worst ones because those don't show stress? Custom Officials are trained to spot signs of stress and some are extremely good at it. I rarely experience delays when I cross the border but, I've seen 2 old ladies being singled-out and surprise, carrying a trunkfull of illegal cigarettes! I go by no problems and they stopped 2 old ladies!
Again last week, I drove by Canadian Customs in Prescott, with my motorcycle. Quick chat with the officer, remove the full face so he can see I match my papers and I'm on my way! I push my bike to the search area, so I don't block the way while I but my helmet back and get my papers in my wallet. Behind me, an older gentlemen with a nice Mercedes. He had to pull aside and got searched! Me, a "biker", no problems and him, a respectable old man, getting pulled aside. It's all in the attitude and appearance of guilt.