The key to layering is to start with the softest layer and work your way out to the most durable. Begin with a light undershirt, add a top shirt, then build up. Add a vest in a suiting fabric, or maybe a wool cardigan and then finish with some weather-resistant outerwear. That way, nothing is too bulky and the progression makes sense.
Mix your fabrics, textures and colors. Bold sweaters (be it graphics like Fair Isle or bright cable knits in shades like mustard or cerulean) aren't so shocking when they merely peek out from under a jacket. And when worn with a shirt and tie, the look is downright dignified. On the other hand, a chunky cardigan looks relaxed when teamed with a striped Tee.
Speaking of contrast, this is the perfect opportunity to work casual pieces in with more formal items. A crewneck sweatshirt is an ideal piece to layer over a shirt and tie, paired with a blazer. Wear your beat-up jeans under a nice shawl collar cardigan with some dress shoes. Or knot up your best silk knit tie with a plaid flannel shirt.
Without a doubt, the easiest piece to work into any outfit is a vest. From down-filled puffer vests and tweed waistcoats to knit sweater vests, they add texture and color without compromising any comfort. Under a blazer, a denim jacket or even a military coat—a vest just works.