This layer, and those thereafter, should not fit tight, but fitted to your body. Opt for polypropylene long underwear so it stays put beneath the other layers. Again, no cotton long johns here! Non-absorbance is vital in this layer.
This layer is also significant, as it works by trapping cold air and moving it away from your body. The fit of these items is important. Here, you'll put on waterproof trousers, and a synthetic fabric shirt and sweater.
We need a solid amount of thickness to really insulate the body from the piercing chill of ECW. If you do not expect to get wet, down jackets and parkas are a wonderful option. However, they can become heavy and unconducive when wet. Use synthetic fabrics if that will be the case.
This is the final layer for your body. The wind shell will ideally help seal everything up in a nice bubble of warmth. These jackets significantly increase the percentage of heat concealed in the layers, making them necessary additions to your getup.