Tip 1: Delve in. It is a good idea to try to do a good deep clean at least quarterly. Don't be afraid to really get in there and pull everything out. You could be surprised what you find.
Tip 2: Purge. Whenever you clean it is always important to reevaluate whether or not items are necessary. It is also a good idea to get rid of broken or unused toys following holidays or birthdays to make room for the new. Make sure to have desigated bins: donate, sell, and trash. It makes the process go quicker.
Tip 3: Clearly define spaces. I remember once hearing that just like we have addresses for our homes, every item in your home needs to have it's very own address; a place that is designated just for it. Make a special home for school papers, jewelry, books, stuffed animals, different pieces of clothing, shoes, laundry, treasures, DS games, toys, cds, craft supplies, backpacks, etc.
Tip 4: Include your child in the process. It will give them more ownership of their space, teach them how to part with items, and hopefully help them to keep a tidier room.