Socks...wool or synthetic, like Thorlos

Underwear...I like Capilene (Patagonia) or REI's "MTS" underwear.

Pants...I used to wear Levi's for summer backpacking. I still think they're OK for summer in many places, but they're heavy. There are lots of nylon "travel pants" out there. They're thinner than I like, but many people seem to like them. For me, the ideal would be jeans made of medium-weight nylon or polyester, but I haven't found any yet.

Shirts...the nylon "travel shirts" abound. "Viyella" shirts, usually 50/50 wool/cotton are nice, but I don't know if they're available except maybe occasionally at the Goodwill. Pendleton makes nice lightweight wool shirts. The wind, of course, blows right through both. An old cotton/poly dress shirt is probably as good as anything for summer.