That’s pretty much my solution. Since we live halfway across the country, there are (thankfully) few occasions where I have to address the in-laws directly. When I do, first names only. I’ve only seen them something like 5 times in the last 17 years.
DH refers to my parents by their first names. That’s how they introduced themselves, and that’s how everyone is comfortable.
A cab to the airport!
I always use first names when addressing my MIL and FIL.
We get along great and I love them to pieces, but I have a Mom and Dad already, thankyouverymuch.
I’m slightly more concerned about whether or not my stepmother-to-be will insist I start calling her “Mom” once she and my dad make it official later this year. So far, the subject has never come up, but she’s acted progressively more motherly each time I’ve seen her… I’ve been very gentle in my rebuffs, but that sort of request will probably push me over the edge.
I don’t speak Chinese or Taiwanese, and she doesn’t speak English. She knows a little Japanese, so I call her okaasan although that may switch to obaachan now that Beta-chan will be here soon.
My wife’s friends can sympathize with Hokkaido Brit. It’s often part of Asian culture for MILs to lord themselves over their DIL, and even worse, for the husband’s sister to give the wife a hard tim.
I’ve heard quite a few horror stories. 
My FIL is a different story, I call him by his first name. We share raunchy jokes - last year, for Christmas, he asked for drill bits — which I presented to him embedded in a pair of tawdry high heels decorated with sequins and feathers.
I call her by her first name. I’m not her child, so she ain’t ‘Mum’, and she’s not my superior, so she ain’t ‘mrs x’. We’re both grown adults, so I treat her on equal terms.
I called my ex-MIL Mrs. Lastname. But she was elderly, and very much from an older generation where the use of that title meant respect. I like her very much, though.
My current MIL, who is younger and from a more informal generation, I call by her first name.
In Dutch, the problem isn’t so much with by which name a MIL is adressed, but with what prefix. English only knows “you”, and that can be used to adress everybody from the Queen to one’s kids. In Dutch, we have “u”, which, like the French “vous” or the German “Sie”, indicates formality, respect and distance. Jij (German “du”, French “tu”) is more informal.
Try avoiding to use one of those while adressing someone!
My ex called my dad: “Wanja”, which was how we called him when we were kids instead of “Daddy”. But his dad died before he met me, so there was a vacancy for the title. 
Whenever she signs Christmas presents to us, she does it,
Love
Mom/FirstName
One year she forgot the slash and it just said, ‘‘MomFirstName’’
So that’s what I call her. ‘‘MomFirstName.’’ Two syllables, in one breath.
It’s silly but it fits. I mostly adore her and she mostly adores me.
So, you use jij, then, for your MIL? Do some people use u?
I actually don’t call her anything. I never have a need to call her on the phone and when we’re in person I just look at her and start saying what I have to say, precisely because I don’t know what I should call her.
Yup, I’ve solved this problem nicely.
I call her Mom. I love her, respect her and thank my lucky stars to have such a great MIL; and I know she loves me as one of her own, but I still feel a little awkward calling her Mom. This after 29 years of marriage to her son.
I lucked out and got 2 sets of in-laws. I call my step-mother-in-law and my father-in-law Mom and Dad, because that’s how they sign cards and e-mails to me. I call my mother-in-law and step-father-in-law by their first names, because that’s how they sign cards and e-mails to me.
I did, however, ask my own Mom if it would offend her if I call my SMIL “mom,” and my Mom told me to go right ahead, she had no problem with it at all.
See, now, I can definitely see it bothering me. If my kids end up calling their MILs mom, I won’t make a stink about it, but I don’t think I’ll like it!
Me, too.
My wife and I both call our MIL’s Shirley.
Yes I am serious.