I've been a fabric collector since long before I started actively quilting. If I see a fabric that "speaks my name" it almost always comes home with me, either right then or when I go back for it. Enough times of going back and it being gone and you know what happens. . . See it, like/love it, buy it.
I may not have a specific pattern on mind when I get it, but I usually have at least a vague plan for it.
I mentally divide all fabrics into two big categories - focus fabric or coordinating fabric. It's really easy to have way too many focus fabrics and not enough coordinating fabrics, so lately I've been consciously seeking ones that will coordinate with what I already have.
I'm finding that pale to light shades of blenders, marbles, and the like are harder to locate, especially in batiks. And those fabrics are the ones I run out of first.