I agree with Jas: looking at your MIL in a broader sense (imagining her childhood experience) can help. This is my main strategy with dealing with my own mother with grace. On the surface my mother is a very difficult person to like... but once you put this behaviour into the context of her life experience, especially during her formative childhood years, it helps to explain why she is the way she is. It doesn't EXCUSE her... just explains it... subtle but important difference.

Other than that it help to know yourself... acknowledge your weaknesses so they don't sneak up on you. I know that personally I have my weak spots. Hearing people make broad sweeping statements about mothers (judging them) and the raising of children (harsh methods) can really get my blood boiling. But as I have grown older I notice the symptoms of anger within me earlier... and I address them. I start to deliberatley pause before speaking. Take extra care to chose my words carefully. Also I remind myself that many people actually feel more comfortable in a conflict situation that a calm one. I know it sounds strange. A book called Families and How to Survive Them taught me this... it was a pivotal moment in developing grace under fire!!!

I would accept that there will always be people like your MIL who push your buttons. Try to get yourself into a mindset that you actually welcome the challenge of getting along with her for the sake of peace within your family. Of course you don't have to pretend that you like her... but you can get real good at showing her a basic level of respect. With a bit of luck she will, on some level, acknowledge this and reflect this respect back at you. Learn to use the phrase "lets agree to disagree then" I have found it really useful. And don't forget to use eye contact instead of words if the moment is getting tricky. Sometimes a certain look (not aggressive, more a look that says "I wonder what you are trying to say?" does the trick. In a power struggle the dominant person will use cues like body language and eye contact to assert control... by keeping and maintaining eye contact you are conveying a refusal to bow down. Make sure that you are thinking respectful thoughts at the time though! Goodluck!