Not sure about documentation, as that seems to be in a state of flux (more on the coming home side) and changes monthly.
But NE IA is spot on about your attitude, going in either direction. Answer their questions honestly and don't try to buddy up to them with a joke answer.
I've applied those rules without fail dealing with the Canadians going in. Coming back, my experience has varied.
On one occasion coming back, I had an officer hit me wtih a fast barrage of questions, which started at, "Are you carrying any citrus fruits?" I wanted to remark upon the reputation of the sweetness of the tangerines from the famed Nova Scotia citrus groves, but bit my tongue and answered with a very direct, "No". The barrage, several questions later, wound up with , "Any grenades, bombs, tanks, or missiles?" I was in a Honda Prelude, and told him, "Just the .50 cal you see mounted on the roof." His question was facetious enough that I got away with it, but there are other guys that would have come unhinged at an answer like that.
On another occasion, I was detailed to break up an ugly impasse between a postmaster (a well-known character about town, but a flaming arsehole) and his letter carriers in a border town, about 100 miles from home. A friend of mine whose family loves their tea knew that what passes for Red Rose in Canada is a different animal from what they sell in the States, and asked if I could pick up some of the Canadian variety for them. So I took my lunch break one day and popped over the bridge and got a couple of boxes for him. Coming back, I got the usual questions. Residence? Citizenship? How long have you been in Canada? Well, it had only been about twenty minutes, so the purpose of the trip was the next question. "To buy some tea." Still in auto-mode and like he hadn't heard me, he asks if I'm bringing back anything with me. "Two pounds of tea bags." That snapped him out of auto-mode, and he inquired why I might have driven 100 miles one-way for two pounds of tea. I left my friend's preferences for Canadian tea out (put it out as my own preference), and explained that I hadn't driven that far for that purpose alone, that it was a matter of convenience as I was in fact in town on official government business, and simply popped over to a Canadian grocery whilst I was so near. He asked for my government ID and asked about the nature of my official business. I told him I had been sent to try and clap a lid on the postmaster, Mr. C (names aren't important here). As it turned out, the agent was Mr. C's son-in-law. He was able to stifle his laughter but not his grin. He handed back my documents, wished me all the luck in the world, and waved me through.
Fond memories and fun stories to tell, but don't mess with those guys. Have your paperwork (customs [bill of sale, etc.] and immigration [documntation]) in order and deal with them honestly is the best advice.