Server :: Keep Servers Running In Stable And Secure State?

As far as I know, servers are stable and don't go down easily, but every single server will eventually go down some day, either from hardware/software failure or from hacking.

But as sysadmins, our job is to keep servers running healthy as long as possible.

So I'm conducting another short survey (I might start more survey threads, and thank everyone for kindly replying my previous post):

1. Have you encountered server failures? What's the most common cause for server failure?
2. What is your most important trick in avoiding your server go down?
3. What security rules do you follow to protect your servers?