I have just spent a good chunk of time sitting in the boys' room while they cleaned it up. You'll notice that I was sitting and they were the ones cleaning. This is how I have trained all of my children to pick-up their rooms and it works. Well, it works in that eventually they are actually capable of cleaning up a room, not that they always keep their rooms clean.
If I have not trained a child to clean a room, it is too overwhelming for them if I just send them upstairs with the instructions to clean it up. (There are always some children who come by it naturally, but it really is a learned skill.) This is especially true is the room has exploded. A child will look at the mess and not have the slightest idea of where to start. Consequently, it seems to be a task that will never end, so what's the point of even beginning? So I have learned that I need to help. This help does not involve me picking up at all. I settle myself into a chair, preferably with a cup of coffee, and I direct.
My direction provides the thinking about the problem that they are not capable of yet. I also think out loud in that I share my thought process with them. So, we look at the room together and I suggest that starting by picking up all the Legos would be a good start. My children are usually able to do more and more picking up without direction as the room gets cleaner. And then they tell me that they're done. For the first 'inspection' I usually don't even have to arise from my chair to point out the piles along walls, in corners, and under beds for them to tackle. After the room is sufficiently picked-up we move on to actual cleaning with dust rags and vacuums. When it's all done I help them admire their work by pointing out how well things are picked up and clean.
This is not a one-shot solution to children's cleaning. I figure I probably spend one to two years on this process. It takes a while to develop the skill to see what needs to be picked up and know how to do it. I have watched with each of my children as they focus on the one or two things that might not have a home or they don't know what to do with. I can direct them to put down the hard item and instead focus on the easy ones to begin with. Children also don't see disorder. This doesn't mean they don't appreciate it when things are nice and neat, but that they don't look around a room and say, "Boy, this is messy!" either. They just know when a room doesn't make them happy.
This same philosophy is one I use for other rooms of the house as well. Both J. and I have watched children clean bathrooms, kitchen, or any other room they are responsible for. Watching me clean something only goes so far, they need the experience of actually doing it themselves, but in a way that allows them to succeed.
I have a new article up at Heart of the Matter Online about Oh, the Places You'll Go! Take a look.