A good manager looks after a project not its people, concentrates on the big picture while letting others deal with the small details. A good manager achieves this by delegation.
Assuming you've hired the right people in the first place you should be able to let people get on with their jobs — if you try to do their jobs for them you will fail through lack of expertise or lack of time.
I would add that iteration is also key - a manager should check on a regular basis that what has been planned is what is being done and that if not ensure there is time to change what is being done as early as possible. Good staff and good managers appreciate that some things will take a few iterations to get right but it is better to iterate than to take the first version of everything (and foolish to plan for this) - not iterating leads to over-design and slow progress as everyone desperately tries to second-guess all the situations their work might have to cover.
Iteration is also the best way to get a feel for individual workers' pace and abilities.