Over time I have found that I like to buy one yard at a time. That way I'm not anxious about cutting into it (for fear it will be gone and I'll never have any more of it!).
I find that I do not use the fat quarters I have purchased. It always seems like there is not enough of any one fabric to act as a focus for a quilt.
I have occasionally purchased 3 to 5 yards of a fabric that I think I might want to use as a border; however, I have rarely used them so have stopped doing that. It works better for me to shop for border fabric after the quilt is pieced.
Once in awhile, if a fabric seems awfully expensive or something I won't use much of -- such as a pop of yellow or orange -- I'll buy only half a yard. Once in awhile, if I really like a fabric, I'll buy two yards of it.
I try not to go shopping for fabric anymore, because I have so much in my stash already! For the last few years, the only fabrics I have purchased have been batiks. I gave away a lot of the JoAnn Fabrics type calicos I bought years ago when I started quilting. My tastes have changed a lot over time!