I usually have something in mind when I buy my fabric...either it's for piecing or for backing. For piecing I buy 1/4-1 yard depending on the fabric. Neutrals I need ALWAYS, so I buy in larger amounts. Bolds, brights, novelties, I buy just a little. If I think it'll make a good backing I try to buy 2-5 yards at a time.
I have been quilting for about 8 years and this is what seems to work for me!