I have been quilting since 1998 - I have built my stash by saving the leftovers from previous projects - I have never bought yards of fabric just because I liked it - always afraid I wouldn't be able to find something to go with it later when I wanted to use it. Also, a friend of mine did that and now she has a lot of fabric that she doesn't really like.
I also buy from garage sales and, of course, always take donations! I don't have a huge stash - I sort into shoe-box size boxes according to colors - pink/purple, black/white, tan/cream/brown, yellow/range, blue, green, red ... you get the idea - also boxes for batiks, 30's, civil war, thimbleberries, christmas. When the box gets full and the lid no longer sits down on it, it is time for a quilt from that color.
Thank goodness I LOVE SCRAPPY QUILTS. They have almost become an obsession - now when I finish a project, I cut the leftovers into 1.5", 2.0", 2.5" squares - I am also collecting for a specific project so those sizes are cut if the piece is big enough - I keep a post-a-note on my 12.5" ruler with the sizes I am saving so I don't have to look it up all the time.