Mind your ps and qs

This expression, meaning "be very careful to behave correctly", has been in use from the 17th century on.
Theories include:
an admonishment to children learning to write;
an admonishment to typesetters (who had to look at the letters reversed);
an admonishment to seamen not to soil their navy pea-jackets with their tarred "queues" (pigtails);
"mind your pints and quarts";
"mind your prices and quality";
"mind your pieds and queues"
(either feet and pigtails, or two dancing figures that had to be accurately performed);
the substitution of /p/ for "qu" /kw/ in the
speech of uneducated ancient Romans;
or the confusion by students learning both Latin and Ancient Greek of such cognates as pente
and quintus.

The most plausible explanation is the one given in the latest edition of Collins English Dictionary: an alteration of
"Mind your 'please's and 'thank you's".