I wrote up some thoughts long ago here:
The TL;DR version is:
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above all else, they must participate heavily, and plan to do so for a long time in the future
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even temperament and patience are huge
Beyond that, I strongly advise that you have several moderators and make sure they check with each other on anything difficult. If it is straightforward (obvious spam) then they can obviously handle it alone, but when things get hairy, they should have a policy of checking their response with at least one other mod to make sure they are setting the right tone.