The shoe accessory includes a platform for supporting a conventional shoe. The shoe accessory's base has a convex or other non-planar surface that causes the shoe accessory to be stable in a longitudinal (heel-toe) direction, and unstable in a lateral (medial/lateral) direction. The non-planar surface may be defined by a ridge segment on the platform's bottom side. One or more straps are provided to temporarily secure the wearer's conventional shoe to the platform in a non-destructive manner. A generally U-shaped heel retainer may be provided adjacent a rear portion of the platform for receiving the heel of the shoe. The shoe accessory makes a flat-soled shoe laterally unstable while allowing for longitudinal stability and a naturalistic gait, and thus facilitates exercise of leg and foot muscles during normal walking, etc.