I'm kind of a new quilter, so I'm currently working on my stash as well. This is how I categorize my fabrics.
Civil War
Baby for gifts
1930s
Christmas
Halloween
Thanksgiving
Bright colors: primary and secondary colors
Pastels
Tone on tone ivory and tone on tone white
Black and other dark solids.
After I sorted and organized I could see where the holes were in my stash.
I've been working a little on landscape fabrics; just a dab of brick, stone, grass, wood, sky etc.
And, trying to get a small assortment of florals that can be incorporated into things.
If I really really like something and think I might use it in a large quilt or as a border, I'll buy 3 yards. For just building a stash of maybe use here and there, I'll buy 1-2 yards. That's enough for a small border or binding.