mind one's p's and q's

And the way one has to mind one's P's and Q's is trying to the last degree.

And Uncle Silas was a very "grouchy" man, and one must mind one's P's and Q's, and never be naughty, or Uncle Silas would have the law of one.

To mind one's P's and Q's; to be attentive to the   main chance.

And so the worthy old housekeeper decided that on the whole it would be well to be careful—to mind one's P's and Q's as it were,—to pause before rushing pell-mell into a flood of unpremeditated speech, and to pay the strictest possible attention to her regular duties.