My situation with my MIL is different than most, so I probably shouldn't answer at all. My husband has an unusual relationship with his mom. He was adopted by his grandparents as a young child, and was not raised by his mother. She is legally his sister. And no, she was not an unwed teen mother. She was married, and my husband is the younger sibling of their firstborn. They divorced not long after my husband was born.
Anyway, she is typically very careful to appear to be concilliatory about pretty much anything and everything where my husband is concerned. She knows she screwed up, and he allowed her to have a place in his life. She has pretty dysfunctional relationships with the rest of her kids too (husband's older brother, and the two younger half-siblings).
When husband and I were married, I refused to allow her to make me 2nd to herself. She tried. She was accustomed to blowing into town and "suggesting" she and my husband go out for breakfast, and tried to continue this after we were married (without me). Umm.. No. You will not sleep as a guest in my home, and then leave your spouse, me and my infant child at home while my husband takes you (and only you) out for breakfast. Sorry sista.
I also do not EVER concede the front passenger seat. And I have never, and will never concede my bed to a guest (her or anyone else). There is a guest room/bed. A sofa. The kids can sleep on the floor and you can have their bed. Or if that isn't good enough, you can get a motel at your own expense.
A few small, subtle, statements (deeds not words) will do wonders. But your husband must understand his place in it beforehand. See, my husband would never ASK me to sit in the back so his mom could sit in the front. If your husband doesn't fully agree that YOU are who he clings to and that the rest of his family is "peripheral" now, then no amount of signals or statements will make a hill of beans worth of difference in her treatment or opinion of you.
And yes... I do talk to my spouse about stuff his mom does. And he laughs right along with me.