You have to have a good base layer. The layer in your clothing system that is going to be in constant contact with your skin has to be of good quality, or you'll suffer the consequences in blisters, sore spots from rubbing, and moisture absorption.
For reviews of various base layer products, see here and here.
RailRiders
RailRiders is a favorite among backpackers, and with good reason. Their clothes are lightweight, durable (I can personally attest to this, having lived in their clothes for months at a time), and quick drying.
Patagonia
Patagonia have a well deserved reputation as a quality manufacturer. They're environmentally friendly (if that matters to you), and make a lot of products that tend to perform reliably well.
Mountain Hardwear
Mountain Hardwear is another well liked manufacturer that carries a few different base layer options.
SmartWool
SmartWool makes, not surprisingly, wool products. Wool is a great choice as a base layer, especially in the colder months, because even soaking wet it's still warm. Smartwool also makes some great socks.
IceBreaker
IceBreaker is another wool manufacturer that make a lot of popular base layer products.
Ibex
Ibex is yet another wool manufacturer that sells a selection of base layer products and is popular with hikers.
