I suffer from the same and wouldn't be able to get through the winter without my Plan!
I really, really recommend getting a lightbox. I wouldn't be able to get out of my room if it wasn't for mine - it really is a life-saver. I got mine off of ebay, but you can get them from loads of other outlets - expect to spend about £60-100 for a good one, but I've had mine for years and got it second hand and had no problems with it!
My second stage is sleeping - do'nt know about you, but I have trouble getting to sleep (even though I'm usually in bed by 8pm ¬_¬) and then I can't get up in the morning! So I have tincture of Valerian to help me get to sleep when I'm really having trouble, and then I have Guarana in the morning to help give me a bit of energy if/when I need it.
I also find eating properly helps - I try to have a larger lunch than dinner so that I'm not going to bed feeling over-full and sluggish, as I find that stops me getting to sleep as well.
My final weapon is getting outside! Every day, without fail, I get out for a walk (and don't include the walking to the bus stop in the morning *lol*) Usually at lunch time during work, I make sure I get outside and at least walk around the block. I often use this time to phone parents/friends for a catch-up as well, so I don't get bored!
Not saying that all the above cures it or means that every day is fantastic, btu it really does reduce the number of "duvet days" where I just can't face the world (and helps me keep my job!!)
Hope some of that can help!