It has been said that a person’s house tends to be a reflection of their mind… and I think it’s definitely the case that how a house looks/feels affects its occupants. If you have your house pretty much sorted, that is a feel-good factor and something less to potentially worry about.
Basic cleanliness is good of course, but I’m talking more about clutter and really just things that lie about somewhere because they don’t have their own proper spot. The latter may sound a tad anal, but essentially if you store stuff everything has a particular spot to go so by definition that’s how you organise things. Nothing new there. I’m not saying you’ll be labeling each drawer and box, but some people find that handy also. In any case, I think the key factor is storage, since if you don’t have a spot to put something, of course it’s going to end up just “somewhere”. Having storage for stuff makes it easy to not have clutter, so you’re more inclined to maintain it as well.
By the way, I’ve found this works well for kids also. Little smurfs at around age 2 generally love to be very organised, everything needs to have a spot and so on. If you have sufficient (and suitable) storage space for their toys, having them clean up their stuff becomes really easy and natural. In my house my daughter actually now has the master bedroom, simply because it has the most daylight and floor space of all the bedrooms. I don’t need daylight in my bedroom, so everybody is happy and better organised (I don’t mean to start a revolution in other households, you don’t have to tell yours kids about this ;-).
In Australia there’s a nice chain of stores called “Storage Space” which I sometimes visit for ideas. I tend to not buy much there since it’s quite costly. My personal favourite place for ideas about storage tricks is IKEA. For instance, I’ve always found shelves sucky for clothes, but it’s pretty easy to build something with storage trays that can be pulled out, as well as smart hanging options (for instance for pants).
In closing, a short story that was told to me at Linux.conf.au back in January: a group of friends got together (some travelling interstate) to help a friend clean up their house, sort things. Now that’s a very tricky thing, not everybody would appreciate such “interference”; but I understand they got it right and it really helped that person well beyond just having their house organised. I hope they’ll pop on here some time and tell the full story directly, as I think it’s a great example of how you can, very practically, help someone get back on track. I also think it’s a brilliant example of true friendship.