I have been very aware this year of the impact that gloomy days have on how I feel. I have found that I feel much better when I am in a well lit, relatively warm room. I changed all bulbs in the overhead light in my home office to 23 watt CFLs. I also keep a small electric heater turned on while I am in there, with the thermostat set just high enough that I am slightly warm when wearing long pants, socks and a long sleeve shirt. Anytime that I notice myself losing motivation, or feeling down I can just go into my office for a while, turn on some music and I start feeling better quickly. I also try to uncover my windows whenever I see that is sunny outside.
I also agree with much of what others have suggested, especially exercise. When I exercise in the mornings I can tell a difference throughout the day.