Done properly, the flag should stay up until after mate has been fully and properly executed.  For those without such time management problems, there is an excellent book on mating patterns by GM Kama Sutra.  But the real trick for most of us is in gaining a decisive advantage over a suitable opponent.  Indeed, some of us cannot even find such an opponent -- nevermind worrying about a premature flag-fall, or an overlooked mating attack.  The key is to stay on top of things -- don't let minor tactical problems interfere with your overall strategy to ultimately force mate.

  But don't just take my word for it -- ask the experts who have actually done it!