Clearing stuff out when sharing a home

How do you recommend handling decluttering when part of the clutter in your home is your significant other’s or your children’s?

I’m pretty unsentimental about stuff myself — an amazing gift I’ve received from my parents — so I have no trouble saying goodbye to old, unneeded things.

But my husband keeps a lot of things he collects (musical instruments, vintage video games) or memorabilia (e.g. a whole bag of stuffed animals from his childhood or boxes of course material from our college days) or stacks of papers he doesn’t take the time to organize and sort through.

Up until now I’ve just focused on keeping my stuff trimmed down and organized, and managing my thoughts around his personal things and how he choses to handle them.

Any thoughts?

Thank you!