Monitor Everything
Okay, is anyone tired of my server admin tips yet? Yes? Too bad.
Monitor everything... Put as much info at your finger tips as easily as possible. Put that info in a place where you will always be looking at it for some reason. For example, I made a vBulletin plug-in that monitors 4 memcached servers (including a latency test it runs) as well as 10 blade servers. This shows every time I'm in the admin of the forum (which is a lot), so I can't help but to not see it.
I wrote a little daemon that runs on my servers that can quickly report back whatever info I want (time, disk RAID status, server load, MySQL replication status, etc.)
The more info you have in one place (especially when you run a bunch of servers), the easier it is to see anything wrong. For example, I had an issue with a web server serving requests slow one day... it ultimately ended up not being a problem with the web server, but the memcached server it was using. The latency test was showing ~2,000 ms latency (2 seconds) vs. the normal 0.5 ms (1/2000 of a second).
And be proactive about monitoring stuff... don't wait until something bad happens to start doing it! Then it's too late.

Can I go to bed now?
I'm a boy... from San Diego, CA (USA). And I don't really have a whole lot else to say about myself.