I was a little confused at first because I thought it was supposed to be "OK Google, now [do this]". It should actually be "OK Google Now, [do this]". The way you say it and the length of pause is critical.
I did the training once and have not had any true false hits on it. The few times it has fired I was explaining to someone how it works and forgot that it would actually fire. I also say "Dear Google" in conversation quite a bit to imply that I want to search for something I am talking about with someone. That's the only other phrase I would try to use, but like how it works with OK Google Now [...].