For the most part, three layers: moisture wicking, insulation, and wind/rain blocking. Water conducts cold, so you don’t want to have moisture in your clothing. Select your clothing for its ability to get your body’s moisture out through the layers and into the atmosphere. The greater the number of insulative [del]lawyers[/del] layers, the greater the impingement of mobility, but sometimes it varies due to the coverage/fit of the product or the need to change to less or more insulation during an activity (e.g. layering down when climbing and layering up when the sun goes down). And then there are those times when it is so cold that you wear everything you have (this winter it was seven layers for me on a couple of deep freeze night skis when I forgot my parka at work – but for forgetting my parka, it would only have been three layers because the parka also had it’s own insulating layer on top of my usual middle insulating layer).
If you are going in and out of buildings and vehicles, baggy old-fashioned 24 ounce wool lumberjack pants with suspenders are terrific as somewhat insulating somewhat wind protecting overpants, for they breathe exceptionally well, making it possible to stay warm outside without acting like sauna pants when you are inside or in transit. The downside is that they are heavy, absorb moisture, and are not waterproof[del], and sheep keep trying to mate with you[/del]. I use them a lot when sunny daytime lift area skiing because I sweat when I telemark ski, so the extra breathability of the outside layer is a bonus rather than a drawback.