When preparing for a new game (be it a one shot or a campaign) our party, yours truly included, often slips into a humorous mode. It's usually light-hearted fun, and most of us like it, but some, yours truly included again, find it distracting and disruptive for the mood, after a certain unspecified point. This point is not universal, though, it comes sooner for some and later for others. Also, this doesn't happen only when we meet in person: our preparatory emails usually fall victim to joking as well. It doesn't matter what we're about to play, either: the (introduction of our) most serious stories get as easily derailed as the lighter ones.
What's the best way, in your experience, to cut back on the pre-gaming humor (both live and in preparatory emailing etc) - without completely eradicating it? (Once again, I'm talking about the preparatory phase, mostly. Creating characters, inventing background stories and agreeing on setting details etc.) We'd just have to bring those "enough is enough" points closer to one another's: how should we do it?
I've found a related question which already has some good advice - How to deal with players not taking things seriously? -, but it's not the same problem. We (usually) do take things seriously once actual play begins. The trouble precedes that, which can be a real turn off for some, and the beginnings of our stories suffer for that.