In random order:
Speak as little as possible. Tell her as little as possible about your life - she doesn't care anyway - keep it all very bland and shallow. Show no emotion at any point, except possibly boredom. Stare into space during conversation, and use stock responses to what she says whenever possible (my personal favourites are "oh, how lovely," "oh, dear, how awful," "maybe," and "oh, right.")
Get caller ID so that you don't pick up the phone to her more than you think is absolutely necessary.
Have no qualms about being "rude", i.e. calling her on unacceptable comments or behaviour. It's OK to say, "What a horrible thing to say/do" (bonus points for saying it flatly and expressionlessly). It's OK to say "Goodbye now. I have to go," at any point whatsoever during a visit or a phone conversation.
Show no guilt and never apologise unless you actually have done something wrong (unlikely).
Decide what boundaries suit you, and stick to them. You are the one who decides how much contact you two have, and what type. If she dislikes your boundaries that much, she may go no-contact with you, and save you the bother (though sadly this is unlikely). She will behave exactly as badly as you let her behave. You have nothing to lose and everything to gain.
She will probably behave much more like a human being in front of people who don't know her well. When you have to see her, try to do it with a third party there, too - one she still keeps up appearances in front of.
(Suggestions based on what works for me in a similar situation - hope some of them may be helpful! On re-reading, I can see I just come across as a complete bitch... you'll just have to trust me that I'm actually pretty nice to everyone else in my life!)