You don't. The asymptotic approach of storage costs to 0 has made some people believe that everything is worth saving.
Like Fred Wilson. http://www.avc.com/a_vc/2009/01/confessions-of-a-pack-rat-ak...
Confessions Of A Pack Rat (aka My Document Retention Policy)
You don't. The asymptotic approach of storage costs to 0 has made some people believe that everything is worth saving.