Wool socks

On wool socks:

"That said, the suit-wearing man should usually be donning wool socks year-round. Wool socks do a better job of keeping feet warm in cold weather, and cool in hot weather. This is because wool socks are much superior at wicking water, keeping feet dry and regulating their temperature. All else being equal when it is warm cotton-encased feet will get hotter and sweatier, and when it is cold, they will get colder."

Via

Interested in the Dore Dore 100% Wool Over-The-Calf sock, but ... so expensive! Even on sale at $28/pair.