I only go to the store (G Street Fabric) when their 25% sale is on.
I stroll through the stacks/shelves and see what I'm attracted to. Sometimes it's a new collection; sometimes it's a particularly lovely mix/blend of colors.
I also force myself to seek colors that I am not usually drawn to but that complement those colors that I am drawn to.
I put out bolts and bolts of fabric on the cutting table and I mix them around until I "see" a vision for what I want to create-- at least in terms of color and print. I have NO idea at this time what the quilt will look like.
I purchase what I hope will be enough (I'm getting better at this), take it home, throw it in the wash, fold it up, add it to my stash. At these times, I usually take out my entire stash (no where near as large as the quilters here) and re-arrange what I've got into new groupings, seeking "just the right" combo. I may then go through my various quilting books, and the patterns I've saved on my computer and see if anything jumps out at me.
Most of the time, I group the fabrics for the selected project, and then put everything away.
Sheesh... writing this makes me laugh pretty hard.