I still have a hard time with this also. I don't usually buy them all at once. I will buy fabric that I love that I don't have a project for. Then i use from my stash or when I decide on a project go looking.
I seem to not get either the right contrast or the color that will make it pop and provide movement and end up with a blah quilt.
when I am stuck though, I ask someone who makes quilts I admire.