It varies. I used to go up all the time with nothing more than my drivers license. A couple trips ago they told me (when returning to the US) that I needed to get a passport or a certified birth certificate (not the hospital one) before they'd allow me back in again.
I've had my vehicle (and myself) searched a couple times, but I expect that with the amount of travel up there I was doing. Once I got my passport they haven't said another word about it.
Oh, something obvious, but I'll mention it just because. If you happen to have a weapon in your car or on your person usually, remember to keep it at home. They're notoriously picky about that. (And yes, I forgot to leave my handgun at home once...I had to drive back down to seattle to leave it at a friends house)