I've told my extreme-packrat DH before that I'm just going to purge his stuff if he doesn't do it. He never does (he keeps every $2 receipt, crumpled in pockets of course - and wants to hang on to T-shirts with huge holes in them, etc.) so on the rare, rare occasion where I actually do some decluttering, I just throw out the completely useless or simply nasty stuff. Then later I tell him about it and he's always fine with it. Of course I'd never throw out anything of real sentimental value.
He does the same thing for me, actually - I'm not quite as bad of a packrat, but I assign 'sentimental value' to everything and so have trouble parting with items that really need to go. For example, my grandma who is now deceased gave us a toaster oven for our wedding 5 years ago. It conked on us about a year ago and I just couldn't get rid of it, so it sat and sat on the counter. DH finally just took it and got rid of it without telling me he was going to do so. I was hugely relieved when he told me...I knew that we had to get rid of it but I just couldn't do it.
I don't know if this dynamic would work for everyone, but we both understand each other's clutter tendencies and so we trust each other to help us through our hardest decluttering issues!
But we have come to this understanding over years of practice. When we got married my DH's biggest piece of packrat clutter was...an 1984 Mazda
. It would barely run and was just gross, but he parked it in our designated parking spot for 2 years while we had to search for street parking to park our usable car!
: He finally got rid of it (donated it) and cried the day he did! But it took 2 years of that car sitting there, windows broken, obviously never going to be driven again...for him to decide on his own to junk it. I should have just arranged the pickup myself; he'd probably have been relieved like I was with the toaster oven.