How do you handle the upgrades on your live site?
Do you backup, test run and close the site, etc?

What's your protocol to ensure as little downtime as possible?

I never test a new software, regardless of what it does, until it has been released for at least 2 weeks. This can allow me to look into the bugs reported, the fixes, and the general publics idea's of the software.

Once I have decided to upgrade, I promise the users a timely upgrade, close the board, and begin. I personally am more worried about offering a quality forum than having a 5 minute upgrade. I will take the time necessary to insure that things went well.

If it takes longer than expected, I will send out an email to ensure I haven't given up, or taken a break, but that I am still working contently. I also email the users afterward to announce the reopening, as well as the protocol for bug reports.