or everyone has to watch their
> ps and qs (what does that mean anyhow,
> always wondered).

For many, especially on the Crossfire Board, I would plant my tongue in my cheek and mutter that it means APPEASE and ACCUSE.

But here's the standard meaning:

MIND YOUR PS AND QS
A puzzling and quirky idiom

There has recently been a discussion on the Usenet newsgroup alt.usage.english about this somewhat outdated saying. As so often happens, a series of posters have recounted the explanations they've heard about its origins without really advancing the discussion. It would be difficult to reach a conclusion in any case, as the facts to base it on have been lost. All we can do is theorise, which can be fun, but it does tend to generate more heat than light. Here, for what its worth, are the facts so far as I know them.
Its meaning in recent times - and the one I learned as a child in west London nearly 50 years ago - has been "to mind one's manners", "to behave properly". This is a weakened sense to the one it had in the nineteenth century, when it meant, according to Eric Partridge in A Dictionary of Historical Slang: "to be careful, exact, or prudent in behaviour".

These are some of the explanations I've seen advanced in various places:

Advice to a child learning its letters to be careful not to mix up the handwritten lower-case letters p and q.

Similar advice to a printer's apprentice, for whom the backward-facing metal type letters would be especially confusing.

Jocular, or perhaps deadly serious, advice to a barman not to confuse the letters p and q on the tally slate, on which the letters stood for the pints and quarts consumed "on tick" by the patrons.

An abbreviation of mind your please's and thank-you's.

Instructions from a French dancing master to be sure to perform the dance figures pieds and queues accurately.

An admonishment to seamen not to soil their navy pea-jackets with their tarred queues, that is, their pigtails.
It is possible to put forward objections to all of these. Why should p and q be singled out for attention in handwriting, when similar problems occur with b and d? This comment might be thought to apply with even greater force to the poor printer's apprentice. The pints and quarts explanation sounds reasonable, provided that men in bars used to drink beer by the quart, as in fact they did. The French dancing-master explanation sounds just too far-fetched to be credible, as does the one about the seamen. The mind your please's and thank-you's seems just as unlikely as the others, but is seriously advanced by some dictionaries, the current edition of the Collins English Dictionary among them.
There are two similar usages recorded:

There was once an expression P and Q, often written pee and kew, which was a seventeenth-century colloquial expression for "prime quality". This later became a dialect expression (the English Dialect Dictionary reports it in Victorian times from Shropshire and Herefordshire). OED2 has a citation from Rowlands' Knave of Harts of 1612:
"Bring in a quart of Maligo, right true: And looke, you Rogue, that it be Pee and Kew."
Nobody is really sure what either P or Q stood for. To say they're the initials of "Prime Quality" seems to be folk etymology, because surely that would make "PQ" rather than "P and Q".

Partridge says that the phrase learn your Ps and Qs, was common about 1820, again being advice to children who may be confused about the two letters.
You may feel the first of these tends to confuse the issue rather than illuminate it, and you may be right. It may just be coincidence. However, the second does tend to support the idea that it relates to children learning their alphabets. If I had to make a choice, I'd plump for the alphabet-learning origin.
What we do know is that mind your Ps and Qs was first recorded in 1779 but that it is slowly dying out. To lose it would be a pity, as it is a link to the past and makes a good subject for some quiet speculation and ingenious attempts at explanation. In common with so many words and phrases in English, its origins must remain a mystery.