This what i saw on a large forum, and this is what i have learnt from it.
1. Don't have Too Many Mods
A big board what I was on had over 50 moderators this is too many. It wouldn't up being a board within a board. Moderators are picked because they are heavily active in the board. The moderators were too busy fighting amongst themselves to properly run the board. The board I was on has literally lost dozens of fantastic moderators to in-fighting. All of these could have been avoided if the owner had stepped in in a timely manner.
2. Stay Involved
The owner of the board in question basically quit being involved with the board. He decided that he wanted the board a source of income rather than a hobby. There is nothing wrong with that, in fact, I think that what of most forum owners want. However, You need to stay in touch with the board.
3. Keep Your Mod's Informed
Mod's can't do their job if they are not aware of "special circumstances". If you make a special exception allowing a user to "break the rules" mods need to know that. If they don't...don't blame them for moderating that person.
4. Spell Out Your Rules Clearly
Rules are rules. Mods have nothing else to go by. When rules are loosely followed, moderators are stuck and don't know what to do. During my time on the board,i saw the users trying to find a way out of the rules.
5. Talk To Your Mods
If you have an issue with a moderator, approach them and discuss it. The biggest way to loose a good moderator is to react when you should inform. If you think a moderator is acting inappropriately send them a PM to clear things up.
6. Respect Your Mods
Never, ever, trash your mods in public. See number 5. If you think a moderator has wronged you, you need to clear the air with them in private, not in public. In public, support your mods.
7. Give Your Mods Breaks
On an active forum,with mods, sometimes they get too involved. Let them have a break when they need one.
8. Be careful of over moderation
On a board with 50 moderators, you cannot afford to allow every moderator to remove whatever they want. You're just asking for cries of over-moderation. The board I was on did exactly that. If someone found offense with a post it was removed, period. At some point you have to say "hey this is ok". Otherwise your forums are reduced to only hello threads.
9. Respect Your Past
The board I was on was taken over by a third party. 3 years later, there is exactly 1 original moderator left. The rest have been either removed or driven out. If a board is popular it is popular because of the staff.You have to remember that the board you bought got popular in the first place because of a good staff.
10. Last point
Respect. You have to treat your staff the same way you would tree an employee in real life. Go back through the points. If you were a manager in a company, they would all fit. Treat your forum as a business with employee's (mods) and customers (users).