I can’t believe I forgot this:
Test, Test, Test
And when you’re done testing, test some more. You can’t assume that you know how something works. You need to know how it works. Test it. Verify your database design by testing. Check your queries by testing them. Be sure the new index works by testing it. Test the new deployment process before you announce it to the development (way before). When you get done with all this testing… Start testing again.