Most of the border-security people act like regular folks and make it clear that they're just trying to do their jobs in a reasonable and friendly way. But one does get the occasional [...] who is out to prove something. Or not really proving something, just someone who for whatever reason has decided to be more zealous than the average. That has a way of dampening my enthusiasm for crossing the border. And yes it does make me kind of nervous.
Just make sure you know who you are, who you're with, where you're going, what goods you're carrying, especially anything which resembles alcohol, tobacco or firearms, and how much it cost. Don't try to B.S. them, they meet 1000s of people every day and they can spot a liar from miles away.
Think of it from their point of view: they're doing their job if they can find people attempting to cross the border who are illegal immigrants, criminals, or who are importing stuff without giving the government what they (the government) consider to be their fair share. Make it easy for them to determine as quickly as possible that you do not fall into any of those categories and you'll be their friend.
It will save a lot of discussion and argument if you keep receipts for the stuff that you're bringing back into Canada ... once I purchased liquor in Delaware (tax free) that was only something like $10-11 dollars per 60-oz bottle and the tax collection guy gave me a very skeptical look when I told him this, and furthermore that I had lost the receipts. He let it go but I vowed to be more careful about receipts next time.