I suffered from it for years, until I changed my diet. I cut out added sugar, caffeine and bad carbs and upped my intake of fresh fruits and veg. Also, I take a multi-vitamin, a B complex and try to get plenty of Omega 3's. It helps to stay in a routine, to go to bed and get up at the same times, even if you don't have to. Good cardio exercise helps too, it's easy to forget it's winter when you're sweating.
If I feel myself getting sucked back into SAD, I have a few last resort tricks I do:
-get a haircut or highlights
-go to a tanning bed
-watch a movie about summer
-turn up the music and dance
-go someplace that's just silly fun, like a bowling alley, party supply store or Chuck E Cheese's
Sugar. Ack. I go back and forth on this one. I stay away from it for a few weeks, then slip back into it for a few weeks. Come to think of it, I've been "in" it for the past few weeks. Maybe a contribution?
Suddenly I'm in the mood for bowling!

I like the idea of a regular exercise class. I stopped with the personal trainer just before my surgery in October, and haven't picked up on it again. I'm sure the lack of regular exercise has added to it all. (Gee, ya think?!) Some things start to seem so obvious when examined under different light.