Ok, here's my contribution to the site. Take everything here with a grain of salt. This stuff will seem high with the *duh* factor, but can be overlooked for the newbs...

#1: ALWAYS, ALWAYS, ALWAYS backup ALL your databases. Practice restoring them to a new database while leaving the current in tact. Setup a local machine and practice rebuilding on a different server. This is also handy if (like me) you move servers and have to get everything running as quickly as possible.

#2: Learn some basics about db's databases.about.com is a great place to start - not too techy, but good info. If you use forums frequently, reading this type of documentation will keep you from getting flamed when you ask a stupid question.

#3: Here's somewhat of a rare case, but worth mentioning seeing as I spent a few hundred hours tracking this down. When your site gets huge (we now have over 50,000 users registered) and you move to a server cluster: MySql likes to do a reverse DNS check. DNS settings are out of the scope here, but this is what happened to me. The MySQL box was not seeing the reverse DNS to the web server - or at least the web server wasn't responding fast enough. I had a dedicated line run between them - same problem. After hours of scouring everywhere (and about 7 days of intermittant connectivity issues) I found that configuring a static route in the /etc/hosts file fixed the issue. The MySQL dev team said "oh yeah, it does a reverse check now - sorry we forgot that in the documentation".

#4: Don't work on your database when you are exhausted, it WILL lead to trouble. I have a regular 9-5 job, and run the site after work. I stop working at 11:00, regardless. I've had too many late nights that lead to even later nights fixing what I had broke because I couldn't think anymore. Not everyone has this luxury, but I'm a one-man-show on the server..

Cheers all, and good luck.