Let's start at the bottom. Stefani's "Haru" shoes come from her own line, gx by Gwen Stefani (available at ShoeDazzle). And, like her L.A.M.B. bag, they're comprised of three horizontal straps in varying widths. But, with her jeans are cuffed so they hit right above her ankle, essentially her pants look like a fourth strap. Who knows where her shoes end and her pants begin? Not us. (At least it's not entirely obvious when you look at the ensemble as a whole.) Combined with the stripes created by her belt, that rad stripe (or censor bar?) across her chest (by Bailey44, and available here), and rectangular sunglasses, Gwen is a regular stepladder of cool.
The trick to keeping this multi-lined look in "polished, casual" and out of "messy" territory is sticking to a cohesive color scheme — in this case, good ol' red, white, and blue. Keep that in mind the next time you start creating patterns where there are none. Or, rather, earning your stripes.