1. Set aside time when you can work on the project together. Even if you only have a couple of hours on a Sunday afternoon, get together with your partner to at least start working on the project. Once you've got the project going and you've set some parameters for what you will keep and what you will purge, you can work on it individually.
2. If you don't work well together, break up your home into zones and go your separate ways. Send your other half out to the garage to organize stuff while you work in the attic.
3. Create separate piles to help whittle down your stuff. By creating three piles – trash, donate, and keep – you can make short work of going through a lot of stuff.
4. Be prepared to be the voice of reason. If your girlfriend refuses to get rid of her collection of stuffed animals, try to reason with her. If reason doesn't work, try the "I'll get rid of this thing you hate, if you'll do the same" approach.
5. On the other hand, be willing to give a little. You may not understand why your boyfriend is keeping that collection of concert t-shirts he outgrew sometime around middle age but it’s not worth an argument. Besides, I'm sure there are things he thinks you're unreasonable for keeping!