It's been a while, but here's a new excerpt from my theater set design book Alice Through the Proscenium, from the "Design Elements" chapter:
Line – A strong horizon line, maybe, or the stripes of columns or floor. Lines are straight or curved, vertical, horizontal, or diagonal. Every edge of the set creates a line you need to understand. Composition. One thing to beware is a diagonal across the stage which pulls the audience’s eyes back to… what? A nasty sucking void? Or the family refrigerator-shrine?
Pattern – That black and white checkered floor, that plaid sofa. Control it. Strong pattern distracts. Or becomes a joke as in The Invisible Circus, where the performer wore a tapestry coat and blended with the tapestry wall… until he moved. Play games with pattern: establish a black and white grid like a checkered marble floor, then turn a few squares into puzzle pieces or ducks.