Planning ahead is probably the most important thing to remember in asking for a paper extension. In most pressing situations, like in the case of having a full-time job or game days (football players would know this), that professors will grant you a paper extension if you ask ahead. If you ask for a paper extension 12 hours before it's due, chances are your professor will think you just procrastinated and came up with an illegitimate excuse. Don't be that person.