I don't have a very solid system. I have a notebook of graph paper where I draw out any plan that I want to do, in that, I calculate dimensions, yardage, pieces and blocks require, etc. I usually work in that book when I am actually planning a quilt, have fabrics and an idea of what I want to do. When I know I want to group certain fabrics together for a quilt, but don't yet have a pattern, I put those fabrics in a zippered plastic bag that sheet sets come in. When I DO have a pattern decided, but just not started yet, I will put the pattern or my notes in the bag with the fabric. I save links, download "some day" patterns and save pictures and keep them all in a folder on my computer.
Sometimes I just grab fabrics and go with the flow.