On long-haul Himalayan treks like the Annapurna Circuit or Everest Base Camp, layering is everything. And you're going to want to love your base layer because you're going to wear it like a second skin.
Duofold mid-weight wool Veritherm top
I packed three each of the very affordable Duofold mid-weight wool Veritherm top (crew, $41; mock zip; $46) and bottom ($41) and rotated them on a daily basis. The pieces never chafed, rarely smelled, felt comfortable under layers and not too claustrophobic when they doubled as my pajamas in my -20-mummy bag. In lieu of washing these base layers in the water scarce high-Himalayan environment, I'd just shake them out every morning and rotate to my next pair.
Another mandatory item you'll want is a Buff. Trekking in the inevitable wind and dust can wear you out and cause a low-grade respiratory illness called the Khumbu Cough. One way to try to stave it off is to cover your nose and mouth with the 100 percent seamless, 100 percent microfiber Original Buff ($22; planetbuff.com). Plus, if you have a bad hair day, as I always do, you can cover your mop with the Buff, which comes in at least a few dozen patterns. This handy tube also does triple duty as a handkerchief, but you might want to wash it out before you put it back on your head.
Finally, if you're climbing high, you're going to want the warmest, most comfortable down jacket available. I love Rab's Infinity down jacket ($280; rab.uk.com). The Infinity's shell is made with a shiny, almost luminescent, ultralight, super-strong Pertex Quantum GL fabric, which allows the 850-fill European goose down to loft to its maximum potential. The roomy hood allows for a helmet, and the jacket comes with a stuff sack, so you can tuck it away in your pack until you need it.