Backup, backup, backup.
Restore, restore, restore.
In other words, don't even try upgrading a live server until you have confirmed that you can restore the original content.
There are 3 types of backup you can perform.
- Product backup (from the SharePoint UI)
- STSADM backup of site collections
- SQL Backup of content databases
I would do all 3 of them before an upgrade.
In a perfect world, I would build a virtual server based SharePoint server and restore the content from one of the 3 methods (3 is the fastest if you have more than about 10Gb of content). I would then try the upgrade on this virtual environment.
Just because the upgrade works on a virtual environment doesn't mean that it will work on your production platform because there may be some differences, especially if you production server has been in place for a long time and other things have been installed on it.
I have had a 50% success rate with the upgrade process so I'm well versed in the restore process.
Even after a successful upgrade (from previous version or B2TR), I would do all 3 backups again, rebuild the server and restore the content so that I had a 'clean' environment moving forward. This is where you might find that a side by side upgrade leaves some dependencies on the old 60 hive and you may dread ever having used that application template or FrontPage. Pay close attention to the prescan log file as an indication of how clean you upgrade will be.
You have been warned! Don't try the upgrade without 100% confidence that you CAN restore (in practice, not theory).