I have basics that I keep available (Kona Cotton Black, white on whites, creams) that can be used with anything. I get a variety of those if they are on sale. After that I look for things that jump up and down on the shelf and call my name. A lot of fabrics know my name, especially batiks. Sometimes an idea for a quilt will pop into my head while I'm shopping. I like scrappy quilts so 1-2 yards of a fabric is usually enough, unless it will be used for a border or sashing.