"Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away." -Antoine de Saint-Exupery
Purge: Toss all the excess. Rid your life of all the unnecessary things, thoughts and unwanted feelings. Essentially, this is the first step. As your space gets clearer and more open, you'll realize that something larger than just the obvious physical aspect is changing. You are changing too. It is a beautiful revelation.
Pursue hobbies that are fun and inexpensive: Find leisures and creative pursuits that don't require buying/storing a lot of stuff. There are numerous activities out there waiting to be dabbled on. You just have to find what best suits your interest.
Reduce media exposure: Often times, we don't even realize that we want a new pair of shoes unless we see it on television or magazines. That's advertising and marketing tricking us into believing our pair of beige-y nude pumps must be replaced by a blush-tone nude this season. It's like being lured to enter a buffet restaurant at times when you don't even realize you're hungry.
Stay away from the mall as much as possible: Be gentle with yourself, if you know you're more likely to be tempted, stay away. Eventually, the wanting of things will go away. Until then, find other ways to cure boredom.
Listen to your heart: There are no rules when it comes to simplifying your life. No one else can set the limits for you. What is "enough" for others may mean something else to you. Don't let anybody tell you that you're doing it wrong. Everything you need to know about the subject, you already know. You just have to listen to that inner small voice that speaks the truth.