I used to be a bit of a hoarder; probably inherited it from my mother. I got over it by reading a bunch of simple/minimalist/environmental/frugal living blogs (they all kinda overlap and form this big anti-consumerist cluster). If you are interested in the subject, here are some good articles you can read.
Things You Own End Up Owning You
Visualizing How Your Stuff Owns You
How Clutter Costs You Money
Minimalism 101
The Simple Life
How I Live My Simple Life
The Library Is Your Friend
Cult Of less: Living Out Of A Hard Drive
Minimalist Fun: The 100 Things Challenge
100 Ways to Simplify Your Life (And Make Yourself Happier)