These are informal guidelines for new administrators, based on what I've picked up over the last two years.

• You are a janitor. You've been issued a mop and a bucket. Part of being a janitor is to clear up messes–not cause them. Ideally, your actions will never be noticed, save by the grateful individual who sees that his floor is now clean.
• You are not smarter for having the mop. If anything, you've just become dumber, because you've acquired a whole new range of responsibilities and a commensurately low level of experience in exercising them.
• You are still an editor. Administrators wear two hats. Some, like Essjay (talk) or Mackensen (talk), wear even more. When you edit articles you're a regular user. When you block somebody you're an administrator. It's important that the two persona do not cross the streams–you should never find yourself pulling rank in a content dispute.
• You will be wrong sometimes. Taking this into consideration, mind your words and actions because you might have to retract them and apologize later.
• You are a mediator. You will be asked to informally mediate disputes because you have a mop and bucket. Your primary goal should be directing efforts towards talk pages. When there's an actual content dispute, favor article protection over blocking. This focuses attention on the thing that matters most.