Layers! Thin, wickable layers, and a windproof outer jacket. I'm taking a set of long underwear and my running capris (will double as running pants for some workouts on the onboard treadmill as well as an underlayer for expeditions), two pairs of good hiking pants, a couple of long-sleeved shirts, two fleece overlayers, good socks, mulitple pairs of gloves and glove liners, and hiking shoes. If my experience in Iceland last year is any indicator, the main concern is keeping out wind and rain, more than bundling up for warmth. Fortunately, there is a good laundry service on board, but not much storage space in our cabins, so we're being encouraged to pack lightly and take advantage of the laundry.