The insulating layer helps you retain by trapping air close to your body. Polyester fleece vests, jackets, and tights are classic examples of insulation ideal for outdoor activities. They not only trap air but are also made with moisture-wicking fibers to help keep you dry.
Natural fibers such as wool and goose down are excellent insulators. Wool sweaters and shirts (especially the new generation of merino wool products) offer soft, reliable warmth and keep on insulating even when wet.