An Idea that would work for me, but might not work for others.
Set a date, mark it on the calendar, tell me about it a couple of times. Make it at least three weeks in the future. Other dates after that one can be about two weeks apart. Make sure we or I don’t have anything else planned that day, but hold me to that once the date is set.
“On Friday the 26th, we’re going to go through that desk in the basement and clean it out.” Just one small area, not an entire room.
That day comes. You drag my butt down to the basement desk with one hand, a garbage bag in the other hand. Start picking through things one at a time. If it’s obviously non-useful, like the manual for the car we no longer own, simply name it and toss it in the bag. Get me started doing the same thing. In a half hour, maximum, we’ll have cleaned the damned thing out.
Do not bitch, do not whine, do not make me feel guilty, or I’m going to be unhappy and not want to participate. In fact, I recommend that it be handled as a positive thing and celebrated, at least in a small way, after the fact. That’s positive reinforcement that makes the next time go smoother.
In fact, if you deal with it positively and get it done quickly, it would then be a fair deal to point out other, close by areas (say, the closet next to the desk) and say “Why don’t we hit that closet now, while we’re at it, then when we’re done, we can go out for a beer?”
Just don’t push it too far. If the packrat is getting tired or annoyed or making noises about being done already, leave it for the next scheduled day.
Hell, if your packrat can be bribed (sex, a night out), that might work just as well without the advanced scheduling. Especially during periods of boredom. I know that if I was wandering around the house and asked “What do you want to do today?” and my SO said “Why don’t we clean out the front closet, then have some fun?”, I’d be all over it. (Whereas getting overly ambitious, as in “Let’s clean out the basement” wouldn’t quite get the same response, because it’s TOO MUCH WORK.)
I also tend to think that going through things together is a lot better than telling me to look at it before you toss it, because that can come across as negative or guilting, and/or make me worried about what else might get tossed when I’m not looking.