There are few career moments as exciting -- and these days, as perilous -- as taking over the top job at a company, business unit, or department. But what exactly do you do once you're in charge? How do you jumpstart growth in a slow-growth environment? How do you clean up the mess you inherited? How do you unleash big ideas in cautious times? From the CEO of a high-profile software company to the new owners of a 127-year-old restaurant, four leaders offer 8 tactics to make a sudden impact.