A while back, I realized that I wanted a nice digital SLR camera. Before I bought it, I made a list of all the things that I wanted to do with it. Ultimately, this ended up being a sort of self-justification for buying it, but it did serve one deeper purpose: I now had a list of about 20 things that I had committed to doing with my new camera. So while it didn't actually prevent me from being rampantly consumerist, I did have a whole set of worthwhile things to do with my new camera!
I've also done this for many other things.