Mouth swabs are by far the easiest to pass. I've passed a hair test before, took hours of prep time, lol.
The reason i was able to take a mouth swab instead of a piss test was because I told them I had a problem and can't piss in front of people (supervised psis test, obviously). They pissed and moaned and told me i "had to", but i told them there wasn't shit I could do and they could violate me if that's what had to happen. I had to go back in front of the judge but he said I could just do mouth swabs instead.
I can piss in front of people, btw, I was just trying to find a way to keep on smoking, lol.