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.
Best Answer
I don't have much experience with play-by-email games, so these are probably less useful in that context, I'm afraid.
Don't be subtle.
For any of these strategies, tell the players straight up what you're doing and why. Solicit their opinions and ideas, and create space for feedback about the techniques once they're being used. This creates an atmosphere of trust and mutual accompaniment toward mutual goals, instead of a sense of manipulation and mistrust. The best social engineering is the kind where you enlist the targets as willing participants.
Start earlier.
Provide a time and space for the group to socialize, catch up on what happened that week, and get the giggles out before you try to do anything game-related. How much time this requires will depend on your group.
Provide transition cues.
Playing music during the pre-game socialization and then turning it off, or starting up game-specific music when it's time to settle down, can be very effective. Other techniques include having everyone wash their hands (no, really, it works!), moving from one room to another, saying a prayer, ringing a bell... you get the idea. Be creative and find something that suits the reality of your group.
Don't forget to transition out of the game at the end, too!
If necessary, impose gentle carrot-and-stick.
The nature of the reward and punishment will depend on the group, and this requires group buy-in or it'll seem arbitrary and mean-spirited. But if you can get it right, rewarding the group for staying on-task and gently reprimanding those who don't can be effective. But don't lay this down from on high as the GM; this needs to be implemented by the collective or they'll balk.
Remember that rewards and reprimands can be very simple: often simply having behavior called attention to provides the requisite pride or shame. Combine with the transition suggested above: it makes for clear demarcation regarding when the carrot/stick dynamic is or is not in effect.