Here in this site, some versions, I will stick to that one. But you can pick the one you please.

http://wiki.answers.com/Q/What_does_mind_your_p's_and_q's_mean
It goes :

An alternate meaning for this idiom, and the one most commonly accepted as historical fact, dates back to the Olde English Pubs of yester-year. In order to remind the patrons of how much Ale or Mead they had drunk as the night progressed, the bartender would write on a chalk board (or the like) each (P)int and (Q)uart that had been consumed by each patron. It allowed said patrons to keep up with their tab, and prevented disagreements from happening between bar and patron. The term is actually correctly stated as "'Mind' your p's and q's." In this instance it is considered a warning to not get too drunk to pay your tab. Likewise in today's parlance it is a warning to pay attention to anything in general.

Longman Dict. have:
Mind your manners/language/p´s and q´s  - to be careful about what you say or how you behave so that you not offend anyone.

 A more comtemporary, even funny, explanation here:

http://www.urbandictionary.com/define.php?term=P's+and+Q's
2. P's and Q's 313 up, 385 down
    This axiom, regardless of its origins, has been common in post-victorian Britain as an abbreviation of 'to mind your manners' or, more specifically, to say both 'please' (p's) and 'thankyou' (thank-Q).
    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.

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    As an aside, mind your accentuation in Portuguese, it might be (or it might not be) an advantage someday in some exam.  It´s just a wordplay of mine, hehe.