Ever wonder where that saying came from? You're about to go to a job interview; your mom tells you to mind your P's and Q's. Or you're in line for coffee right behind the town gossip; maybe you should button up the juicy talk. It means to watch what you say, to tighten up your program, to hold it together.
Today in lecture we learned that the saying comes from 'minding your pints and quarts," as in watch how much beer you drink. While we typically don't see beer served in quarts anymore, the saying stems from British temperance efforts to get people to drink less. This origin makes sense, because if you watch how much you drink, you won't be a blabbermouth.