Actually depriving them of N the week before isn't all that great a way to go about things. In fact, many growers continue to give some N even during the first ~10 days of flower, JUST until it appears that stretch is slowing down. After that, you do indeed deprive them of N by switching to a bloom fert that has little to no nitrogen in it.
One thing that some growers swear by is an extended 'night' before flowering. 24-36 hours of pure uninterrupted darkness sends the unmistakable signal to the plant, 'it is time to flower... ready... set... [lights come on]... GO!!!!!'