Turn cardboard boxes on their sides so that the opening at the top faces outward and place on closet shelves. These can be used to hold stacks of clothing from toppling over. Lightweight items such as purses or shoes can be placed on top of the boxes.
Boxes with dividers such as those for wine bottles make great organizers for shoes, scarves, rolled sweaters or t-shirts, socks, underwear ..
If needed, mounting tape can be used to secure boxes to each other or to shelving.