I agree with that it's most likely muscle memory. I had a friend with this problem back in high school. I had a problem of my own (don't remember it) and we just vowed to help each other to remember. If I saw her slouching, I would remind her not to and if she saw me, she would remind me. Ask your husband if there's anything that he does/doesn't do that he needs help with reminding. Then help each other. If there's nothing for him, then just ask him to remind you. I would suggest, too, that instead of him saying, "sit up straight!", that he just say "posture" or something. If he's barking a command about it several times a day, you'll start to get annoyed and resentful, most likely. So just ask him if he sees you slouching (which he has obviously noticed) to say "posture". Eventually, you'll notice it a lot yourself and correct yourself. Then, eventually, you'll just stop slouching, hopefully. Hope this helps- it worked for me and my friend.