I didn't build my stash with any sort of deliberation, I just would buy things that I liked (especially if they were on sale). Then I started getting more purposeful when I realized my stash was full of totally random things. Now I try to keep in mind where my stash is "weak" - lately I've been watching out for good deals on blenders in general and white-on-white and white-on-cream in particular. I also have a list a mile long of quilts I want to make, and for some of those I have begun accumulating fabrics. Like I want to do a "skyscraper" quilt, so I've started accumulating fabrics in various shades of grey that have textures that I think might work for a part of a building.
I also buy fabrics on vacation - when I was in Hawaii I bought enough Hawaiian prints to make two quilts. I haven't made them YET and that trip was over a year ago, so they're languishing in my stash until I get around to them.
The best/worst is when I go on a road trip. Then I want to hit every fabric store along the route, and I want to buy *something* at each store. I try to pick a theme so the fabrics "go" together but I'll buy whatever pushes my buttons. I rent on a road trip to Oregon not long ago (to see the Sisters outdoor quilt show) and I bought a ridiculous amount of fabric on that trip, much of it featured birds or celestial themes. So I sort of stuck to a theme but I bought a lot more too.
Right now I'm actually to a point where I have zero desire to add more to my stash. I still do, if I see something I just "can't resist", but I'm not seeking to stash build anymore. Now I need to knuckle down and trap some of these quilts that are flittering around in my head in some fabric! But before that, I have 4 quilts to make for Christmas, and that's coming much faster than I am prepared for!