Reminding someone to "watch his p's and q's" means to pay attention to the details.
Thus the phrase 'watch your p's and q's' has been in use to encourage people to speak politely, especially children, who remember such phrases better than just instructions.
Kid 2: feck off!
Mum: Oi, watch you're p's and q's!
Kid 2: Sorry. Feck off, please.
Whehn things got rowdy, as they tend to do in pubs, you'd normally hear something like "Mind your Pints and Quarts" and a warning or some such. this was shortened to "mind your P's and Q's"
P's = Priorities
Q's = Qualities
In lamens terms: Always remember that you have "priorities" in life, and the "qualities" of your character will help you achieve them...