For a base layer, it’s very hard to beat Patagonia's Capilene 1 Silkweight Stretch Crew ($35). I’ve worn the short-sleeve version (pictured) during two recent multi-day hikes in the Grand Canyon. It kept me cool, dried fast, and, with a UPF rating of 50+, repelled the sun beautifully. And one final thing to note when buying sun-repellent clothing: in some cases, dark dyes block UV rays better than light ones. The tradeoff, of course, is that dark shirts are also hotter in warm weather.