Let's disregard the fashion statement you're making by wearing an old, stretched-out shirt. The most important reason to leave this beloved tee at home is that cotton absorbs sweat and doesn't quickly release it through evaporation. Wearing heavy, wet cotton against your skin can give you the chills and cause irritation. Opt for technologically advanced fabrics that hold moisture away from the body, like CoolMax, Capilene or Dri-FIT. If you aren't comfortable in the clingy fit of these stretchy fabrics, try a double-layer top that offers two shirts in one. One example: A supportive tank made of Play Dry wicking fabric is covered by a looser cotton tee with flattering ruching on the sleeves.