Fabrics That Don't Breathe
Shun any fabrics that lack breathability, such as nylon or leatherette. Add your rubber raincoat or waterproof jacket to this list as well. (Note, however, that some high-quality waterproof jackets, like outerwear made from Gore-Tex, are quite breathable. It all depends on what it's made of.) Less breathable fabrics hold sweat on the skin when it's hot as well as prevent air circulation. You won't feel very fashionable sweating in too-tight, synthetic clothes as your plane rests on the tarmac under the hot sun.
A foolproof way to find breathable clothes for the plane: Stick with moisture-wicking activewear (I recommend Prana) or clothes sold from travel suppliers like Magellan's, which are designed specifically for travel.