your stash will grow naturally with the left overs from your projects.
if you buy an extra 1/2 yard or 1 yard when buying for a project you will be amazed how quickly your stash grows.
fabric multiplies faster than rabbits when you are busy sewing :)
and as you shop if you see something you absolutely love but do not know what you will do with it yet buy as much of it as you can afford- especially if it's something that could make borders- or a great quilt back.