Now if she tells you to do something that turns out to be the best course of action, there are two good responses that I have found:
1. Thank you. Say "thanks for the reminder" if you had intended to do it but forgot, or "thanks, that's a good idea" if it hadn't occured to you previously. Basically, you're treating her words as advice, not orders. You don't take orders from her, but you're happy to listen to and profit from her advice. This mainly works if she is speaking in a civilized tone.
2. Now I can't do it. This is for the times when she speaks to you in a vicious or belittling tone. Tell her in your own words that you would like to do exactly what she was telling you to do, but now that she gave you such a disrespectful order, your own self-respect forbids you to follow it. Then say nothing else to her, and do something else quietly and calmly. This is a good time to catch up on your reading. Don't expect her to respond to your words as a cool-headed, respectful person would... the words are for her to remember and reflect on later. Let some time go by, then if you still feel it necessary, follow her advice (NOT order) and thank her for her advice as in (1) when she is calmer and more respectful.
All of this should be done calmly and respectfully, in a spirit of cooperation with her as equal partners.
a fine and enviable madness, this delusion that all questions have answers, and nothing is beyond the reach of a strong left arm.