I will either pick a fabric that speaks to me/reminds me of someone and choose a design inspiration (or pattern) I think will work with the fabric or choose a design first.
Once you have a design you can break down what kind of fabrics you're looking for -- bright large print, earthy leaf type print, contrasting solid or marble, something with several swirling colors, ect. This will limit your choices into something manageable and make scale decisions for you.
From there find the fabric you think is most important and find the one that you think is going to be hardest. . . Say bright large print and color swirl fabric.
Once you find those two together the others are easier. You know what colors of green will match and what color they have in common that would be a good accent.
This method occasionally will leave you frustrated looking for that perfect piece. I have the luxury of several area shops so I sometimes go to several stores looking for the perfect fabric, but it also gives me freedom to start with my favorite clearance fabric for the majority of the quilt.