Very little control here--I try not to put the same fabric pieces too close together (however, it always happens--even if I use a bazillion fabrics!), I pay attention to value contrast so everything gets to show off, and with string quilts I mix up widths. I often put things that clash together deliberately--I remember reading that the sweetest of perfumes have an elusive, background rancid note which adds to the scent--clashing, if not overdone, adds sparkle!