I've been crossing the border 4-6 times a month for nearly 13 years now. In all that time I have been checked by Canada twice. Once just a general SOP gander at the car, second time when it was 3 am. and I was picking my daughter up at the police station in Delta (her mom was soused, daughter ran to the PD to wait for me).
Don't even think about fibbing to either side...those BC folks will see right though most any half-truth...the US is much tougher since 9/11. Just look them in the eye, tell them where your going and what your going to do WHEN THEY ASK. Don't go volunteering info they are not asking for...it makes them suspicious and want to dig deeper.
On the US side, be sure to have a passport or CERTIFIED birth certificate. A photo-copy is only good if it is a certified copy. Beginning in 2007, ALL people entering the US will be required to have passports.
My motto has always been "Look them in the eye, tell them the truth". Never had a problem.
US side will typically ask:
What is your citizenship?
Where do you live?
Where have you been?
What/why were you there?
What are you bringing back? (that was bought in BC).
On occasion the US will ask:
Is this your car?
Who do you work for?
Canadian side is typically going to ask you your citizenship, where your going and why, and what you will be leaving in Canada while you are there.
Suggestions:
DO NOT be on the cell phone (or anyone in your vehicle) when you reach the border kiosk.
DO NOT have your radio on when at the kiosk.
If you are wearing sun glasses, remove them when you reach the kiosk. They like to see your eyes.
Have your ppwk prepared (give it all to the driver) when you reach the kiosk. You don't want to be digging around while they ask you questions, OR, they pull you in for inspection.
If you pulling a boat, EXPECT that the US side is going to want to inspect it for illegal cargo (drugs, explosives, etc.). It is what we pay them for, and like has been said, these guys/gals bust dozens each year trying to smuggle in all manner of bad things to the US.
Have a positive attitude, be patient and plan for the extra time that will be needed.
If crossing at Blaine, use Exit 275 (truck crossing). It is typically faster at any given time than the main (Peace Arch). Try to go at off-peak times..like 10 AM till about 2 pm. Weekends will suck until school starts again (vacations). Exiting Canada this is Exit 2A off of Hwy 99, or Hwy 15 if coming from Hwy 1.
Expect the exchange rate to be around 20%. $10 US will get you $12 in Canadian. Not that great, really.
Mis dos centavos.
Mike