Have a sense of humor.
If you're in a leadership role, your attitude can set the mood for your entire team. Though you probably don't want to be the office prankster, you also don't want to be so serious as to put your teammates on edge. The ability of humor to lighten a mood and put the people around you at ease simply can't be underestimated—so don't be afraid to joke around, even when the situation may seem bleak.