If you don't care much about the look of your shelves, Instructables user makedo-able shows off how to build modular shelving from cardboard boxes. The shelves require just a few cardboard boxes and Makedo pins to construct.
After a few folds and cuts, you'll have a set of shelves you can keep adding to at any point. They're not exactly the most beautiful shelves in the world, but they'll certainly do the trick if you're just storing stuff in your closet.
Whether you're sick of your bedroom having junk scattered all over it, or you just have no interest in spending money on shelving, head over to Instructables for the full guide.
Makedo Cardboard Storage Unit [Instructables]