I do cheat a little. I have a whole list of "wannadooz" - quilts I want to make. I print the pattern out the "shopping list" - that is, approx how much I need of each fabric. As I collect the fabrics, I staple a swatch to the printout and make a note of how much I have. It's a 5 X 7 three ring binder thingie that fits well in my purse so I have it with me all the time. I also keep my flyers in there so I'm prepared for those impulsive shop stops. It's been a lifesaver as my girls and I collect for our bug/food/candy jar quilts so we remember what we have.
Does it keep me from random purchases? Absolutely not. :)