Okay Free, You have heard and reasonably so about pets, booze and firearms so here is my take on customs. During the day I usually drive with very dark polaroid prescription glasses, when I approach the customs agent,and when I am sure he is looking at me I remove them and put on my clear ones. I look directly at the agent with all normally required paper, passport, vets certificate in hand and visible. I do not speak until spoken to and answer questions directly and with out any unnecessary talk. Mostly yes or no.
We try to the best of our ability to be in total compliance with the customs laws in all countries we visit. My DW has all our legal papers in a folder in the glove box and gets it out before we get to the agent. I get my license out but have never been asked to present it. Passports are almost always asked for going out and back in.
We are what the agents call "Plain White Bread", older, reasonable hair cuts, no facial tattoos and smiling, not false but sincerely like you're having fun and happy to be there. There have only been two times that we have been searched, both very mild, once my wife volunteered the information that she was glad to be back in the US because she just paid an very high price for one lime. Boarded and lime confiscated. It had a little California Grown sticker on it. The other time a Canadian Agent asked about firewood. Of course I had the butt end of some dimensional lumber for fire starting and told the agent I had firewood, gave him the keys for the cabinet and told him which one. He came back and gave me the keys and had a grin saying, you smart ass you know I mean wood with bark on it.
DW broke the rules after he waved us through and because it was a very remote crossing ask about a place to buy lunch and use a real restroom. He told us where he ate lunch and off we went.
There are lot of subtle things you can do to put the agents at ease and all have been posted here. The next thing you need to know and prepare for is a depository for the hundreds of photos you did not plan on taking. I've talked to several folks that "didn't take pictures" who loved finding out about Photobucket, Tinypics and Shutterfly. It is infective.
Have a great trip.