It really boils down to this: R-E-S-P-E-C-T
Make respect a cornerstone from the beginning.
When I helped a young newly married couple merge their households. When they set about sorting, I reminded them that the bottom line is that they loved each other. She did not appreciate his collection of action figures (Star Wars), but to her credit, she offered him a small area to display a couple which she lovingly dusted and cleaned. The rest we boxed and made a spot in the garage for shelves to store them on.
A client wanted to toss a large tote full of her husband's baseball caps in the donate bag. Whoa, Hold it! Yellow flag... personal foul! I advised against it. Even if he wasn't using them, the hats were his and he should decide. I gave him high marks for storing them all together in one container. She shook her head and tossed them. This doesn't bode well for a relationship.
Respect yourself and your goals.
I worked with a gentleman last year who worked hard to clear lots of accumulated clutter. He called me recently to say how much he appreciated what we had done. He was proud to bring someone to his home and even prouder to tell me that clearing the clutter had made room in his life for someone. He wanted me to meet her.
People come with stuff into relationships. If we expect and want someone to respect our stuff, we have to respect theirs. I have to remember if I say anything about my husband's stuff, he will gently remind me that I have a few too many shoes.
What possessions do you and your loved ones struggle with?