"Look at the fangs!"
"Never tell me the odds!"
"Why is the Rum Gone?"
"I cast magic missile at the darkness!"
Keeping everyone at the table focused isn't always easy. Sometimes the mood of the room, especially with long running groups, can make it all too easy to derail things, and send the session into a purgatory of in-jokes and goofing off.
Sometimes (but not often) that's okay. There are times when it's a better idea to can the game, toss in a movie, and do something else.
Other times, you want to keep everyone on track. There are a couple of tricks you can use.
1. Silence - Stop paying attention to the table. Reread your notes, make sure you're set to jump back into the game, but don't say anything until they're ready to get back to it. Alternately, "take 5" and walk away from the table.
2. Roll some dice. Give the table a moment to laugh at the joke, then pick up a couple of d6s or d20s, and make a show of rolling. Smile. Grin even.
3. Random encounter! Have someone kick down the door, and gain free attacks since the PCs are clearly too occupied to have been ready for an attack.
4. XP Penalty - have them lose 10 or 100xp/minute. Count every minute. I don't actually encourage this one... It works, but makes the players rather unhappy.
The other thing to keep in mind is that the players may be getting distracted because they're just not that into the game. If keeping them focused is an ongoing problem, talk to them, and see what's bothering them.