1) Get a second user account - One thing the users watch is how the moderators act. They take their cues from you. If they see a moderator doing things that are against the forum policy they will think it is ok for them to do the same. By having a separate account for general posting it gives you the liberty to be yourself and post what you want while maintaining your moderator account for moderating and enforcing board policy.
2) Post clear forum rules Another thing users do not like is vague forum rules. Users like to know what they can and can not do. Make the lines of demarcation between acceptable and unacceptable behavior clear. If you have a forum rule stating the forum is "family friendly be sure to define what that means. Family's are different and what one might consider acceptable another may not.
3) Users rating users If your board software has the ability for users to rate other users, turn it off. It will only lead to trouble if you leave it on. You will find small groups of users will constantly award points to their friends to slowly build up their rating power. Then, when someone tags them with a negative rating you will find them, and their friends, will descend on this person with a firestorm of negative ratings to the point where the user will feel singled out and attacked (which they are.) So, just leave those systems off of your boards unless you want to deal with the headaches and PMs of users complaining about the system.
4) Know your Users and their sub-culture Often times on a forum you will find a community evolve. This is especially true of popular boards and among gamers. Many users will look at their post count as a form of "experience points and title changes (based on post count) will be seen as "leveling up. Great care should be taken when your board reaches this level of community. If you decide to change things it is important that you let the users know ahead of time so they will be prepared. It's also important that you not fool around with post counts, titles, etc without giving a warning first.
5) What to do with unacceptable posts When you find a user has crossed the line and posted unacceptable material it is very important NOT to delete the post. Move it to a staff-only area of the boards and retain it for archival purposes. In fact, any materials that are offensive or inappropriate should always be moved to a holding area rather then flat out deleted. This allows you to look for patterns of bad behavior and otherwise protect yourself and your staff from trouble. It's easy for a user to complain the "mods are out to get me! But, if you have the posts to back you up the rest of the users will see the truth.
6) Staff Break Room It is very important that you maintain a private forum for moderators to talk. This forum should be there for mods to discuss problem users and other issues that need not be in the public eye. This forum should also never have posts deleted from it. Giving the mods somewhere to talk privately will also negate any need they might have to PM between each other about problem users. You want the information about problems to be in this forum so all can be kept aware of what is going on.
7) Keep your fellow moderators informed Nothing angers a moderator more then finding out late about a problem on the boards. All mods should be given equal power and equal knowledge of problems on your boards. Also, when a mod takes point on rectifying a problem, give them all the support they need. Do not jump in and try to help unless they request help. If a user perceives that the mods are not in agreement on an issue they will try to use that as a wedge between the mods.
8) Have fun If you are not having fun, take a break. Moderating a forum is generally a volunteer effort. If you feel burnt out take a rest from your moderating duties. Drop a message in the staff break room that you are taking a few days off and relax. A burnt-out mod is no good.
Those are my most basic tips for moderators and administrators of web forums. If you have some useful tips please leave a comment.