The last cluttering post really seemed to help people, so here are a few more points I picked up--most of them in the past week.
Organize for the life you have, not the life you want to have. Don't set aside living space for hobbies you don't do or activities you've given up. If you restart them, you can always rearrange your space to reflect that. For now, make your space perfectly fit the life you actually live.
Don't be afraid to rethink how you use space. Just because a spot used to be a reading nook doesn't mean it always has to be a reading nook. If you don't have enough space or your space seems stagnant, think: Could something else fit better here?
Use what you have. You probably have decorations you haven't put up, shelves and organizers you never used, even replacements for broken items that you never put into play. Go through your house and use all the things you never used before. I guarantee that you've already thought about your home's problems, bought solutions, never used them, and totally forgot about them--so make use of all that problem-solving you did.
Beware stagnation. It means something is wrong. If an area has been organized in exactly the same way for over a year, or if an object (that isn't, say, a sofa, though you might want to rethink where to put your sofa, too) has been sitting in the same place for over a year, change it. Take everything out of a stagnant area and put back only the things you used within the last few hours. Everything else goes into a box. As you need things, take them out of the box and arrange them as suits you best at the moment, taking into account only the things already in the space. No, seriously. Don't take anything at all into account except what's already there. As you add objects, periodically do a sweeping reorganization; you'll find yourself leaving things where you first put them, and that's unlikely to be the very best place for them once more things enter the space. After a week, scoot the box farther from the space you're reorganizing. After a month, take the box away and do something with the stuff in it that doesn't involve going, "Oh, I need that!" and putting it back in the space. Ta-da! The area now contains only what you need and use.
Organize first, eliminate later. This is good advice for people who try to fix an entire room and wind up bogged down in the minutiae of the contents of one single box. If deciding what to donate or throw away is difficult for you, leave it until last, and focus on getting your possessions in order.
Label everything. Label drawers, boxes, shelves, organizer sections, magazine holders. Label things that should be perfectly obvious. Label anything that doesn't move fast enough to get away. Label your own pockets. Label everything, because when you don't, organization creep happens. For example, a drawer is supposed to be for paper clips, butterfly clips, and other paper-holding devices. So you put a bunch of them in. A week later, you're looking for a place to put rubber bands, and you come across a drawer that looks like it's for miscellaneous office supplies, so you dump the rubber bands in with the paper clips. A week after that you drop some tacks in, because hey, random office supplies FTW. A week after that you put stamps in, because stamps are office supplies too, right? And then you can't find any of it, because when you need a rubber band, you open the drawer you actually set aside for misc. office supplies. So: Label everything; make the labels as specific as possible; make the contents match the labels; and don't be afraid to rearrange and relabel when your organizational system develops the inevitable holes.
"Because the cat sleeps there" is not an acceptable reason to keep a piece of furniture you hate and don't use. The cat will find someplace else to sleep.
Never accept any organizational system that requires you to "just" move anything out of the way. You won't do it. Any organizational system that requires more effort than "open door, stick in appropriate hole" is doomed to fail, because the small effort of lifting other objects out of the way is enough of a psychological barrier to prevent 90% of humanity from using the system successfully. DO NOT let yourself assume you're in the charmed 10% that can make systems like that work. There's a name for them, and it's "professional organizer."