While this hasn't been a problem I've experienced in some time, I know there are groups out there where the GM or other players get frustrated because one or two players simply won't stay on task. Jokes come from everywhere, people joke about actions, interupt other people, and then apologize and go "sorry, I wouldn't actually do that". Before you know it, you've done what should take about 10 minutes of play, only it has actually been well over an hour.
In situations like this a "You say it, you do it" rule can work wonders. The rule works exactly as it sounds, everything a player says while at the table is taken as either a declaration of action, intent, or what they are actually saying. If they crack a joke, their character cracks a joke. If they say "I tackle him!" then their character actually tackles him. It will generally take a few moments, or at least instances of enforcement, for the group to realize that you are serious. But around the time when someone jokingly mentions dropping their pants and you run with the consequences of them dropping their pants in public they will get the idea.
It is important to remember though that the intent of this rule isn't to stop fun from happening. It is to get people a bit more focused on the game. When what people verbalize is taken as being in game, it helps keep people from speaking out of turn and distracting others from what is going on. However, that also means that you need to be engaging the players properly. Nothing will turn someone off from a game quite as fast as not being engaged in the game, while having to sit there for several hours not saying anything lest it ruin everyone else's fun. So really, when you call for this rule make sure that you yourself are ready to get going with things.
In general, this rule is best suited for serious moments. Times when you don't want distractions, and want everyone focused on the task at hand. Be sure the clearly state when you are putting the rule into effect, and when it is coming out of effect. Also, be sure that you as the GM don't violate it either. Trust me, it will tick people off when they can't say anything for fear of it happening in game, but you can without it being so due to the role as the GM.
With most groups, on most occasions you shouldn't need this rule. That being said, it is there when you need it for some of those slightly more rowdy days.