I have only been working on a stash for a couple of years. I have a mix of yardage and FQs. Here is what I have done and so far is working for me.
Neutrals and Holidays fabric - I purchase 1-3 yards of stuff I like or that I think I would use such as white on white. beige, soft tonals and prints that "call" my name.
I also have bought some FQs - but did that when the local shop had their FQ frenzy. I now have a small stash. I am using my FQs for applique right now but even at that I have had to go out and purchase specific fabric for items on the most recent project.
Another way to get a neat assortment is to participate in a FQ exchange. When I do this, I purchase yardage and cut my own FQs - then I hold on to at least 1/2 to 1 yard of something that I might use as a background, staple or focus fabric and send off the rest.