There are shoes all over the place, the laundry has taken over the floor, the kitchen table is a dumping ground, and you can't park your car in the garage. And you can't stand it. But your partner? Doesn't bother her or him a bit. If you and the person with whom you live perceive and tolerate clutter differently, you can't force them to declutter, but perhaps using music as an analogy will help shift their perspective.
Here's how it works. Play a genre of music that you enjoy, but that your partner really does not like. While you're tapping your foot or humming along and enjoying, your partner is likely annoyed and covering their ears. Gradually make the music louder. Now have him note how it makes him feel.
Here's the point: while you can enjoy the music; he hears noise. Explain that this is how the clutter is for you. While he hardly notices it and it doesn't bother him, for you it is noise and it is loud and you are aggravated by it...the same way he or she was aggravated by the loud unenjoyable music.
You are not asking them to declutter because they want to or because they think it's important, but rather because they love you and you think it's important.
If you are in this situation, try this experiment and let me know if it helps the other person understand.
Bonus: I really enjoyed this post in Wired on using technology to organize your life as a couple. From calendars to projects to reminders, here's how to get in sync.