A retainer employs a flexible arm with an internal rib. In another aspect of the present invention, a retainer has at least one generally triangularly shaped projection laterally extending from an arm. A further aspect of the present invention retainer provides a laterally enlarged member, a trim panel-retaining head and stem, a central post, one or more flexible wings flexibly spanning between a tip of the post and the laterally enlarged member, and an internal rib and/or at least a pair of generally angular extensions flexibly extending from each arm.