Sun, rain, snow - Mother Nature isn't always going to create the ideal weather for your outdoor training session, so make sure you're adequately prepared for the weather with comfortable base layer clothes. Base layer clothing is slim and stretchy for a figure-forming fit beneath your regular workout clothes. Base layers can be considered tops or bottoms, and some come with proprietary fabric technologies to keep you comfortable while you work out. Look for heat-trapping fabrics that wick away moisture, like Under Armour ColdGear, to ensure you'll stay warm in dropping temperatures and feel dry when you break a sweat, as well as anti-odor properties for extra freshness. Since thermal underwear sits snugly against the skin, you may want to find items that boast multidirectional stretch for uninhibited movement, as well as frictionless, ergonomic seams that won't leave you feeling chafed by the time you hit the showers.