Managing work versus managing people
One final point before we start to dig into the meat and potatoes. It's worthwhile to make a distinction between managing work and managing people. Managing work is actually pretty easy; it's mostly a question of being organized and of keeping on top of deadlines. In today's environment, though, you rarely have the luxury of simply managing work; you have to manage people as well. Managers who step into a management role thinking that it's just a question of getting the work done are riding for a fall; you have to be prepared to deal with the people side of things as well.
Incidentally, this is a mistake that people sometimes make when they're hiring: they hire someone who's really good at getting work done, thinking that this is a qualification for management. But the hard part of management is dealing with people. Never forget that. (By the way, sometimes people make the opposite mistake as well: they hire someone with people skills who's no good at the work. Needless to say, that doesn't work either.)