I have a phrase, it is quite useful: "Can I get that in Email?"
If the answer is "no", then I assume I am free to ignore that request. Since they have no record of the request, then they have ability to fire me for not following said request. It is really easy to play that game, you just have to play along. The issue is, you have to play it 100% of the time.
Good advice. One other, never ever delete an email. Ever.
I've had several occasions in different companies where my ass was on the line but, my ability to dredge out a 2 year old email where I was specifically told not to do X, has ended the discussion pretty quickly.