My #1 way to be careful with a live database? Don't touch it. :)
Backups can undo damage that you inflict on the database, but you're still likely to introduce negative side effects during that span of time.
No matter how solid you think the script you're working with is, run it through a test cycle. Even if a "test cycle" means running the script against your own instance of the database, make sure you do it. It's much better to introduce defects on your local box than a production environment.
