I'll start:
I get flat sheets from the thrift store to use as backing for quilts. They're large enough they don't require a seam, and usually much less expensive than fabric from the LQS.

Ill hit the thrift stores to find 100%cotton clothing with unique and odd designs....

I don't have any good tips, but I can't wait to see what else there is!!! :)

Garage and estate sales.

I use "bad" fabric (too thin, etc) as the backing on doing the foundation squares for things like strip quilts, and crazy quilts, and etc.

I never purchase fabric nor batting unless it is on sale and then I buy quite a bit so I have enough to last a year. Example, I use black to accent a lot of my blocks so when it was 50% off, I got ten yards. Yipes! Yet I know I will use it up. I also watch the free table at Quilting Guild very closely. Word as gotten around that I am the scrap lady--I use almost anything for string quilts for the battered women's center in our county.

I love that you created this thread. I can use some tips.

Use what you have and learn to love and appreciate scrap quilts. I think they're the prettiest anyway. And try not to cut up all your clothing! ;)

Find someplace that sells name brand fabric from a year or so ago at way low prices. Just today my DH drove me to a place 100 miles from here where I got RJR, Windham, etc for $3.29 a yard. Even if you have to buy online and can't 'feel' the fabric. If you stick with name brands there should be no problems.
Unless of course you HAVE to have the $13 a yard stuff that just came out....

Keep all your leftovers for a scrap quilt.

stay out of the fabric stores

let it be known that you LOVE donations (the down side is that you will also get totally unsuitable stuff, too)