In chinese medicine and daoist practices winter is considered a time of energetic restraint and dormancy.
While in the west we suffer from the winter blues, SAD (seasonal affective disorder), chilliness, sniffles, and the like, in chinese tradition winter is a time to rest and consolidate our energy so we that we can emerge refreshed for a new cycle in the spring. The winter blues are a result of our body's fight with its natural rhythm!
Sleep: allow more hours of sleep during the winter. Not fighting with the natural impulse to sleep more will allow your energy to be maximized during waking hours. Do not resort to extra caffeine, sugar and stimulants to force the body to "perform". Eventually these strategies will backfire and result in lower mood and energy. This accumulated fatigue will also lower your immune system making you susceptible to colds and injuries.
Food: emphasize warm to very warm foods, both in temperature and energetically. While fresh vegetables and fruits are always welcome in every diet, balance them out with warming spices such as carrot ginger dressing, small amounts of freshly pressed garlic, a dash of cayenne - such as honey cinnamon cayenne drizzle for fresh fruit. Gravitate more towards soups and broths, cooked vegetables, baked root vegetables and squash, freshly cooked whole grains and small amounts of fish and meat. Make ginger and cinnamon your friends!
Activities: continue your regular exercise and activity regimen but do not push if your body reacts to the cold with stiffness and decreased performance. Winter is a time to maintain a routine, not make dramatic gains in performance. If you feel you need to decrease your activity level, do so, knowing that you are using this time to rest and restore strength.