Hot, wet, or cold, you will rarely find me racing without merino wool on my back (and butt, legs, and feet). The super-fine wool is a magic material that regulates like a second skin—it keeps you warm when the air is cold, and it's breathable and anti-odor for long treks on hot days. Bonus: Merino stays warm when wet, and because it's naturally anti-stink, T-shirts and base-layers can be worn for days. Merino tops from Ibex and Icebreaker have been lifesavers for me in the wilds on long races where I don the same clothing for up to a week straight. Start your merino wardrobe with a short-sleeve like Ibex's Balance T, a tight-fitting top that has proven indispensible for me on a dozen events.
Price: Ibex Balance T, $75