Kids can be real packrats, but beating back the piles of clutter often helps improve their ability to get stuff done in a timely way. Storage devices in all manner of shapes, sizes and models can usually be found for pretty cheap, and they're a good way to get stuff out of sight and out of mind. Same thing with closet organizers and under-the-bed bins.
Then it's a matter of deciding what stays and what goes, and sorting the keepers into categories. This helps your child remember where everything goes and makes it easier to put it all away. It's fun to save old artwork, but keep it fresh by regularly changing what's displayed and then tucking it away in a box for safe-keeping. The same goes for A+ papers and other projects -- these can just add to the confusion when something current goes missing. Help your child develop a system for dealing with items they might need later, but can do without for the moment.