You can choose items with a chameleon personality. The items can change as your child's tastes change. For example, your child's Mickey Mouse comforter can be replaced with a Star Trek comforter, but the bed frame will stay the same. The dolls and blocks decorating a plain book shelf can be put away and trophies and computer games can be added. There are wonderful storage bins available now that will organize your child's books and toys.
Plastic crates work well too.
She also loves being able to act like a BIG girl by cleaning up all by herself. The closet is another spot that deserves planning. Put your child's clothes at a level that he can reach. This will encourage him to dress himself and to put his own clothes away. If your closet is big enough, install a book shelf or drawer unit for additional storage.