I have been buying "theme" fabrics for about a year and a half. I've collected a nice stash that I keep in those three-drawer plastic bins sorted by color. I almost always buy three yards.
Recently I realized that I have lots of "theme" fabrics, but now backgrounds. So now I'm hunting almost exclusively for backgrounds. It's much easier to find those on sale.
When I first started quilting, I purchased the fabric as I needed it for a specific pattern. Now that I have a real stash I'm discovering that it's like the "Field of Dreams." (If you build it, he will come.) If I'm patient, a pattern will come along for each bit of yardage that I have--even the silliest of novelty prints. One of the first fabrics I bought was pink flamingos on a white background. We'd just come back from a diving trip on a tiny island where they had flocks of pink flamingos, and so it spoke to me. The other day I was wondering if I'd ever find a pattern for it. Then a few days later, I bought a book called "Wonderfully Whimsical Quilts," and the first quilt in the book is a pink flamingo applique quilt. It's perfect for my fabric. I even found a perfect pattern for my sock monkeys!
I just love fabric. When I have trouble falling asleep at night, I just think about fabric. It never takes long after that.