How to be in this world but not a part of it?

Lately, I have been thinking about how I spend money. I wish I had the answers, to what is for me a difficult situation. Money is one of those things that you can't do without and yet it is so easy to over do it. I feel sucked into the consumerism- justifying what I have and what I want to more of. Honestly, I don't typically sit at home and day dream about the things I want or need. I often feel content with all that I have but when I am out and about I want, want, want.... It is like when I am out I forget about the overabundance that I have at home. I become unattached of the plush world I live in and reason that I must have more. When I get home and realize that my closet is overflowing, I am embarrassed and I promise myself I won't buy any more but the cycle inevitably starts again. When I went to Africa I didn't think about what I was wearing or if I fit in, I spent my time with the people and enjoying them.
So I am left asking myself how do I consume and not buy into the consumeristic mentality- for me it is a slippery slope! When can I get the things I like even if it means spending a little more? How do I enjoy the things I have and remain content even when I go out? It is so easy for me to find joy in things! I also see shopping as a stress reliever- I used to be able to window shop and not buy but somehow I buy into society's messages that "more is better" and "that my old stuff isn't good enough" or that "I woudl look so cute and I deserve to look cute" etc... Some of the people I admire most are those who have an abundance but who live mindfully of God and others. Their focus is not things but relationships.