The law of 6x
It's a commonly held law of ideas. For someone to "invest" in what you are selling (a night out clubbing, an idea, a record) they need to get exposed to it six times.
How about adding a little bit on the end of that.
...unless your idea is six times better than everything else, and it can spread six times more quickly.
For more on this see Unleashing the Ideavirus (it's not a clubbing book though).