KISS Method (Keep It Simple Stupid - no offense)

Remain age appropriate and height considerate.

Use smaller hangers or just drawer systems (Laundry Tips)

Place items where you (and they) want them. Place the items they access most at the correct height, and prime real estate for their space. Use tall areas of the closet and shelves for rarely or limited used items.  

• You and they get to decide together what gets prime real estate.

For safety, attach necessary items to the wall if your child is at the age they will climb.   

Invest in at least one NICE Square Storage Unit that can hold at least 12x12 square storage containers. I recommend the IKEA Kallax variety, because they are extremely sturdy. You can get other places/styles but pay attention to quality! (Toy Tips)   

Avoid putting lids on containers.

• Once they are old enough, THEY get to choose the sorting method. I KNOW this is hard but TRUST me, if they sort by how they use the items not how we think they should sort, they will be able to clean up better.

Create labels based on age (pictures, words, or both). Make sure labels are easy to change or recreate!  

ZONE items in the room based on use; books together, toys together, arts and crafts together, clothes together (best they can be). Even create a homework / study zone.

Be sure to have a laundry bin in the room!

Take a picture of what the room SHOULD look like when clean so the child has a frame of reference.  

Let your kids decide what is in their room/closet. If they are done with an item, but you want to keep it for sentimental reasons, it goes into YOUR storage area/memory. Also, teach kids at a young age that when they get new things, old things may need donated and that they are blessing others less fortunate.