You will get a lot of different answers here. And you can use the search for even more ideas.
I have learned that if I buy a collection or coordinates for a specific quilt, that I need to keep them together with the pattern and tie them with a ribbon. Otherwise I might use a piece that I can't replace when I am ready to make that quilt or project.
My "stash" I group by theme, juvenile, holiday, etc. Others will sort by color. You need to figure out what works for you.