If you have more than 15 minutes to plan for "emergency shut down" of your servers, I'd recommend:

1. All applications that are running have corresponding startup and shutdown scripts in rc.*
2. Identify the order that your hosts should be shutdown in - NIS or LDAP should shutdown last, NFS servers second last... NTP would go first...
3. Write scripts.
You need to send a wall to all users connected informing them of an impending outage.
You need to ensure that you send the right shutdown options to the right OS types.
You need to create for each command that you are sending - for audit and CYA purposes later.
4. Inform your business/clients/users that these are the "emergency shutdown" procedures. Get them to sign-off and buy into them. If they have special requirements, amend your policy to include those.

Ensure that you have adequate time to shutdown storage devices that may have a great deal of data in cache. Ensure that you have adequate time to shutdown tape storage systems, as the robotics may need more time to get to "home" than you might expect.

There is no shortage of things that you could do, but this should get you started.