If heavy starching isn't enough, you can back the fabrics with a lightweight interfacing or MistyFuse. However, this adds to your cost. I do think the idea to use them to practice FMQ is a better idea.

When I was last in the states, I found some wonderful all cotton sheets at a thrift store for $1.50 each. They were very high quality sheets and not at all "worn out" and wonderfully white--very, very white--and not thin at all. I've been using them in some of my charity quilts and I love them. Plus, I found about 6 or 7 of them --all the same, so that meant I had a lot of that fabric. (They weren't the kind that is so tightly woven that you can't quilt them, either. I've experienced that as well! That is something else to watch for.)

Just be picky about what you choose--you can find good stuff that is sent to the thrift stores. A lot depends on where they get the donations.

I bought a sheet from Goodwill for $1.67. It was a full size cotton/poly sheet. I used it for the backing on a utility quilt. We will use it during the summer for camping, picnics and drive in movies. I think it will hold up to the abuse better than cotton.

I doubt my quilts will be used for anything other then a bedspread, so I'm going to look again for a better quality sheets and if I can't find anything - then it'll become a UFO (I think that's what its called)...in the meantime I'll practise actual quilting on what I have...but I'll pay closer attention next time I'm in the bedding section

Yes you have to be a bit picky with so called "good buys" but most fabric can have some use left in them. Practicing quilting on some of the more well loved buys can still have a use, such as pads for garden seats, car covers and other utilitarian uses. You learn your quilting techniques without breaking the bank and still find a use for them, BTW Fur-babies love our practice bits n pieces :)

A top tip, a pair of pillowcases, un-stitched will give you a full yard of fabric, approximately 72" wide! Absolutely super for binding as you get 14 yards from ONE pillowcase. Don't worry if you get Poly/cotton mix as they are super for children s and other heavy usage quilts as they are made to launder a lot. My charity quilts are all backed with them.

I'd use the more threadbare ones possibly inside (as batting) as long as you plan on quilting it quite a bit (to hold everything in place)....they make good batting, and are more protected inside. I'd have no problem with used sheets IF the thread count was high enough (and not well worn) for backing for more simple quilts (like scrappy utility quilts).
Great idea to use them to practice FMQ, too.

I work at Kohls and right now the flannel sheet sets are on clearance. I bought a king size set. The flannel is pretty and nice and thick. I like to use it for backing scrappy quilts. The king sized sheet set was $11.99 and I had a 30% off coupon.... that makes it $8.40...less than a yard of fabric. I know its not as good as the ones from thrift shops at a few bucks... but it is brand new! I can back one large quilt and maybe a couple of small ones as well!

Some times old is too old to use. It is great to RRR. As you learn more about material you will be more able to discern quality in fabric. It is a learning experience.

Another subject with a wide range of opinion is the use of sheets in quilts for the backing. Be careful with this sometimes they work and sometimes not.

The main reason being that the thread count in sheets is much higher than in standard quilting cottons and it can make quilting harder. If you machine quilt sheeting material the needle can tear the sheet threads rather than pushing through the threads. It can also cause distortion on the quilt top.

It is a choice and many will say they never have a problem using sheets. This is just a heads up on what can happen.

Don't get discouraged over the fabric you thought was a wonderful find. This one time the fabric was very thin.

A long time ago someone told me that if I could see my hand shadow behind the material it was too thin to use. I have used this test many times and I think my quilts are better for doing it.

Passing up beautiful fabric is hard to do sometimes but in the long run there will be other beautiful fabric in the future.
peace