Our guild had a speaker who used clothing for her quilts and presentation. I was very impressed, and you couldn't tell the fabric from new.
In her presentation, she stressed to cut around the worn spots--elbows on shirts, ect. She suggested, as someone has already said, to use the big skirts that maybe have been worn one time.
I see lots of women buying clothing at Salvation Army in our town and making quilts from them.
I haven't made anything yet with old clothing, but I know that you are on the right track. My Mom made a lot of quilts from old clothing. In the Depression and long after, that was all they had for quilting. I have a quilt she made in Pinwheels she told me was from dresses she didn't wear any more, my oldest sister's dresses. That quilt means a lot to me.
Good luck!
Mariah.