1. It's all about the layers.
This is true in all types of travel, long-term and short, but especially for trekking into high altitudes. Temperatures can change very drastically during the course of a day. I always prefer to have an extra layer in my bag than to go cold.
Even if the days are warm at low altitude, nights may still be chilly. On summit days you'll often need to pile on everything you have to get to the top, only to peel it off layer by layer as you descend.
2. Rest and sleeping clothes.
I learned this from the folks at Erratic Rock in Puerto Natales near Torres del Paine National Park in Chile. They called the yucky, stinky clothes you'll find yourself wearing every day until the very end your uniform. In light of this — and even if you are going minimalist — try to include an extra set of night clothes to change into at the end of the day. These clothes will be dry (quite important if you've hit snow or rain that day), comfortable and relatively clean. I usually pack an extra t-shirt, pajama pants and socks. I'll further layer other clothes on top to stay warm at night. Regardless, the layer closest to my skin is dry and relatively fresh.
Oh, the little joys while on the trekking trail.
This technique also gives your wet and stinky clothes a chance to dry and air out overnight. The next morning you can slip back into your trekking clothes — yes, your uniform — and you'll be ready to go.
3. Never skimp on sun protection.
As you advance higher in elevation, the sun becomes scary strong. So even if you tan beautifully on the beach without any sunscreen, be sure to pack ample and [easyazon_link keywords=”sunscreen” locale=”US” tag=”uncormarke-20″]strong sunscreen[/easyazon_link]. Carry a hat that will protect your face from the sun (think rollable foldable sun or jungle hat — we don't need to look pretty while trekking). Trekking with sunburn — head, face or hands — is miserable. And if your sunburn is bad enough, you'll almost feel flu-like. Not good for peak performance.
Also be sure to have sunglasses with quality lenses that protect your eyes. Otherwise, they too will become burned and sore.