I don't know why I don't think of it more often, but rounding the corners of a square or rectangular element within a card design also adds interest.
Stamps: doily (Magenta), sentiment (Verve)
Papers: white cardstock (Georgia Pacific), patterned papers (BG Perhaps)
Inks: Memories Crimson, Versafine Onyx, Colorbox Antique Pewter
Accesssories: fabric brad (Sandylion), paper flowers (Prima), pewter brad and velvet brad (MM)