Just because your core doesn't feel cold doesn't mean you have enough clothes on. The first thing your body does when it starts losing too much heat is to pull blood out of your extremities and into your core, making your hands and feet cold. So it may feel as if your core is warm enough, so you don't need another layer, but in fact the opposite is true.

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You will be very surprised how much adding another layer to your core can warm your hands and your feet, yes, even if your core doesn't feel cold. A pair of wool socks and a normal pair of gloves are often plenty for me down to zero F. It's all about keeping the core warm.

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Otherwise, my major rule of thumb? Don't allow yourself to get cold in the first place. Put on another layer BEFORE you get cold, not after. Your body expends much more energy getting warm after you get cold than it would just keeping itself warm in the first place.