Jeanne,
There are some really good ideas here. We're lucky we have this forum to share.
As for me, I started with purchasing only the fabric I needed for a specific pattern.
Then I moved into fat quarters to add as fill-in's and interest to the pattern.
Then I decided to buy half yards, resulting in 2 fat quarters, which cost more than 1 fat quarter but less than 2 fat quarters.
Then I decided to buy a yard and a half, so I would have a square (for little baby blanket backings), plus some left over.
Then I decided I should buy more - if fabric was on sale. I could use it for a focal point for a new pattern. Besides I was planning to retire from my job and I wanted to have plenty in case I couldn't afford it.
At about this time, I really couldn't find any particular piece of fabric - things were too chaotic. Had I had the forum then, I would have started this topic! Anyway, I tried a few things and finally ended up with my lateral filing cabinets.
Now I don't buy much fabric, cause I can find what I've already bought.
And that's how I buy/bought fabric!
Linda