When I go into a quilt shop I always check their sale fabric first. If I can't find it I ask if they have any. Usually sale fabric is 50% off with a 1 yard minimum. If I see something I like I buy that but I never buy fabric just because it is on sale or if I need just a little bit because of the 1 yard minimum. Many of my backing fabric comes from this source.
I usually have a project in mind for future quilt and I look for fabric for that. I made a quilt for my husband with vintage car fabric which is hard to find. It took me several years to find a nice selection. I made a quilt using animal prints (zebra, tiger etc). Again, it took me several years to get my collection together. Same with the music quilt I made.
I collect 30's-40's repro prints so check out if they have that and buy 1/2 yard of something I don't already have. Someday I will use them in a scrappy repro quilt.
I also look for fabric that has lots of different colors in it because they are so versatile in bringing seemingly unrelated fabrics together.
I have only bought precuts once and a pattern only a couple of times. There are so many ideas out there that I'd rather spend money on fabric than a pattern. Again, I don't want to spend the extra money for precuts when I can cut my own and actually enjoy it.
In looking at people's stashes on this site and others I don't have a large stash but have a nice selection of fabric. I don't buy large amounts of fabric. Some buy 4-5 yards of fabric they like where I buy 1/2 yard. I also don't make a lot of large quilts. I like lap size quilts and things like pot holders, place mats and such with intricate patterns that don't take large amounts of fabric. I also love scrappy quilts.