I have been trying this for several years - sometimes more successful than others. Most of the fabric I have bought this year has been to finish quilts that are scrappy to begin with. I have purchased a stash of plaids/stripes ($2/lb. at a yard sale) to make a quilt that I have been drooling over, but if I have $30 into it with shirts I purchased at Goodwill for their fabric I would be surprised. I am lucky that I LOVE scrappy quilts so make lots of them.
I went to a quilt show with some friends this spring and they all walked out with oodles of things they bought - fabric, patterns, kits, etc. I walked out with a fat quarter for a project I was working on. How? My rule of thumb is not to buy fabric just because I like it - it has to have a project in mind - no more patterns, I have 1,000's of them and will never make all of them - no kits unless it's a project I plan on doing in the next 90 days - these rules really keep those impulse buys to a minimum.
I love Bonnie K. Hunter's website (quiltville.com) because she has many free patterns using scraps. I have several of her books and her directions are vey easy to follow. I now cut up my leftovers into her 1.5, 2.0, 2.5 inch squares and always have some ready to sew into any kind of project. Some of them end up as nine patches on the backs of scrappy quilts - if you use some of your scraps on the back you have larger pieces of backing to use in other quilts (sometimes enough for that outside border/binding).
I also save my binding ends by adding each new one to the last one, I have quite a large roll going right now and it is all ready to sew onto my next scrappy quilt.
I saw a quilt made from "crumbs" - whatever they are - so I am hunting for my interfacing to use as a foundation and I am looking at that container of scraps next to my machine that seriously needs to be reduced.
There are so many scrap quilts that I would love to make and not enough time.