Celtic Thunder, I'm with you. After the holidays is when my blues set in. For me, it has to do with the short days and cold weather. I sometimes I suspect I have Seasonal Affective Disorder, simply because sun and light affect me so much.
I've found one thing that happens is a routine -- to make sure the house is bright. Open all the curtains and shades daily, especially if they're on the sunny side of the house. Turn the lights on in the house if it helps -- I always feel a little guilty when I do this, but I look at it as cheap therapy. If I go from room to room working on things and it's bright, I'm much more liable to keep working. If I walk into dark or cold rooms, I just want to curl up and cocoon.
For me, a huge thing is what food I keep in the house. If I commit to always have plenty of healthy food in the house to munch on, I do much much better. Because I know I'm going to munch, I just have to keep baby carrots, celery, low-cal ranch dressing, lean turkey, bananas and other fruit around the kitchen so that there's something waiting for me every time I start wandering and looking for something to distract me.
I agree with others' suggestions about how we need to get out. Perhaps that's something I need to concentrate on this winter as well.