There are a lot of different theories about layering systems. And most of them have some basis in reality. It is important for climbers to look at what others are doing and develop a system that works for them.
The idea behind a layering system is that you start at the base, with some type of synthetic long underwear. Most will wear Schoeller or nylon pants on top of this for their legs and then finish with a shell layer.
The upper body is much the same. After the long underwear, the next level will be some kind of mid-layer. This is often a fleece. Most guides currently use the Patagonia R1 for this layer, but there are many options.
After the middle layer, things become far more "system-based." By system-based, I mean that it depends on what you build the rest of your layering system out of. Personally, I carry another light fleece layer, a Patagonia Houdini, a Patagonia Micro Puff, and a Rab Super Dru Jacket for a shell.
It may seem like I'm promoting Patagonia with this particular article, but that's not the case. These are items that work for me in my system. Other things may work for you in your system.
The following video describes another concept of layering. It is a good demonstration that was developed by a European gear shop. The video is very hard on cotton. And while cotton has it's problems, there are many advantages to it as well.
Cotton works extremely well in warm environments. If you are going to climb in the desert or you believe that you will have a warm alpine ascent, then it is well-worth bringing a cotton t-shirt along. The same properties that make cotton dangerous in cold and wet environments, make it an excellent option for warmer and dryer climates.
That said, if you elect to bring cotton on a trip and the weather turns for the worst, it is important to have an array of high tech clothes that will deal better with such a circumstance.
--Jason D. Martin