My favorite quote on the topic of simplicity is from G. K. Chesterton: “There are two ways to get enough: one is to continue to accumulate more and more. The other is to desire less.”
I first came across this quote during grad school while reading Richard Foster's lesser-known-but-still-classic book Freedom of Simplicity. It was a life-organizing principle for me then, and I try to come back to this notion of desiring less whenever I find myself wanting things I don't really need.
The other concept that helps me corral my acquisitiveness and materialism is the simple idea that we don't need to own something to enjoy it. I don't recall now when this first hit me, or if I came across it in a book or heard it in a sermon or something. But it rang true to me when I realized that I would purchase something and immediately have a sense of buyer's remorse - okay, I own it. Big whoop. Now what? In fact, it's a cultural lie that we need to own something to enjoy it.
My problem is that personality-wise, I'm a collector. When I was a kid, I collected coins, stamps, stickers, buttons, baseball cards, comic books, postcards, matchbooks, patches, pencils, keychains, magnets, rocks, shells, pop cans, action figures, Transformers . . . it never ended. Not surprisingly, my room was a mess. Over the years, I've found myself winnowing my collections down and limiting myself to certain things and not others. I'm currently maintaining a few ongoing collections (autographed IVP books, US Mint proof sets, Olympic pins, Justice League comics) and forgetting about the rest. In other words, I've been trying to practice the spiritual discipline of desiring less.
Of course, even when I try to desire less, stuff keeps on showing up, like T-shirts or coffee mugs from conferences. How many coffee mugs do any of us need, really? At least when we desire less, it's easier to let stuff go.