When people used to misbehave in old time pubs they would be asked to watch there pints and quarts (like stop throwing them around) hence watch your ps and qs
in the days when public house allowed drinkers to run up a tab if the bill was getting a bit high the landlord would say to the punter to mind his Ps & Qs (pints and quarts) and the punter would know he needed to settle the bill