Buy only the fabric you need for a specific project.
When I began making quilts, I bought waaaaaaaay too much fabric for each of them. In part, I didn't know how fabric is measured, but the larger part was that I thought I'd make multiples of successful quilts.
What I've learned over the years is that the most promising, intriguing quilt pattern is the one you haven't made yet!
I'm doing my best to hold to a "no new fabric purchases" pledge while I work through my enormous stash.
ON THE OTHER HAND: If you think you're going to need a whole lot of a specific fabric to complete a quilt you're making, buy all that you need when you see it! Jo~ann Fabrics stocked a particular black-on-black fabric for so many years that I stopped worrying about being able to buy it as I needed it. Alas, it's been completely unavailable for the past 6 months (both in stores and on the website). While I've emailed Jo~ann's to inquire, I'm not expecting a positive response.