I was a CM for the summer way back in 1991 working Main Street Ops - got to drive the cars and bus too - it was really great. I had no training at all, but I think during the interview process, they can tell who is a going to make a good CM and be friendly to the guests.
Having said that, I was told 3 things that helped me stay "cheerful".
1. My boss reminded us that some people save all their life for only one chance to come to DW, travel great distances, etc. And if we are in a bad mood or are rude, it can ruin their entire trip.
2. We were told that if we were having a bad day, to just call in sick, because if we arrived to work looking depressed, angry or just plain NOT cheerful, we would be asked to leave.
3. We were encouraged to report other CMs who were rude or disrespectful to guests. And we were reminded that Disney sometimes had CMs dressed as tourists who roamed the park asking CMs questions to ***** their knowledge and politeness.