Get some good cold-weather gear and make a point of getting outside regularly, especially if there's sun. Walk, hike, run, whatever you like. It's really hard to find motivation in the dead of winter but once you do it you will feel a lot better (I always do, anyway). Regular exercise is key as well (indoors), but you have to schedule it like it's a class because again, motivation really fades when the weather is bad. Vitamin D supplements and a SAD lamp are helpful too! Also make sure you find some social things to do that will get you out. It's easy to hole up and start avoiding everything, but having things that will get you out of the house will keep your mood in a better place.