Forget documentation - focus on automation (Score 1)

I've seen a lot of folks suggesting that you focus on documenting but I take the opposite view: you should actually be working to make documentation unnecessary. Documentation, by its very nature, tends to become obsolete very quickly - and any good IT guy learns to look at what's actually going on in the system rather than relying solely on the documentation. So you want to make your system as self-documenting as possible:

• Set up a good monitoring system - that's a great starting point for your successor because he can deduce what's important by what's being monitored. Get someone else to be involved in monitoring (maybe your boss?) and you've got a second person who knows what is important.
• Use a good naming convention - EmailServer1, TelephoneCloset3 are good names. Ninja, Macbeth, Comp1, etc. are bad.
• Fix DNS so that common names go the right place - typing 'wiki' into the browser should go to the wiki, 'monitoring' should go to the monitoring system, etc.
• Never build machines manually - Use configuration management software, automate builds, make them as push-button as possible.
• Use an web/online Task List - Get in the habit of using task list software (I like PivotalTracker, personally) Giving your successor a list of what you worked on over the last month is really useful. Plus, it's an easy way to sit down with your management and show them what you'll be working on over the next week/month/year.
• Automate those simple tasks you do all the time - Do you find yourself clearing disk space? Running reports? Run them from your management/monitoring console instead - that way, there's at least a script for someone else to read.
• Finally, set up a wiki to document anything that isn't covered above - there will always be something but hopefully it will be a short list.

I've done this three or four times and it's worked every time.