An old expression, "mind your P's and Q's," was often used to remind children to behave. What exactly are "P's and Q's?" David Feldman in his book of trivia, Who Put the Butter in Butterfly?, suggests several possible origins for the phrase. The most likely theory comes from an expression common in 17th and 18th century alehouses. Tavern owners kept tallies of the ale sold on slate boards. If a customer paid for a pint, but drank a quart or an account became overdrawn, the barkeep would warn, "Mind your P's and Q's." It's a little harder to determine how that related to children's behavior, but it was used, and often, by parents as I grew up.
As writers, we have a lot of "p's and q's" to mind. The first to consider is peliminary background work. Look at guidelines and sample issues of magazines to which you may be interested in submitting. Only by doing this market research thoroughly will you know how best to market your work. Too many times editors receive stories too long or the wrong subject matter for their publications. Perhaps the most common mistake is for fiction writers to submit stories to publications which do not publish fiction. Another mistake is made by children's writers submitting to adult markets or vice versa. Do your preliminary work before submitting and you increase your chances of finding a home for your manuscript. This will save you both postage and paper costs. You won't be wasting time, money and supplies on inappropriate submissions.
Years ago, I wrote an article about submitting to the small presses. One of my readers sent a story to one of the editors I mentioned. He wrote to thank me for the exposure but pointed out the reader had sent him a story of 13,000 words - way too long for his publication. He, of course, rejected the story. If she had done her preliminary research, she would have read his guidelines and known his story requirements. Don't be hasty; take your time and do your homework first.