If cloudy or rainy days leave you in a sad or lethargic mood, get some colored light bulbs and have them shining in the lamps near your windows. They will trick your brain into thinking it's not so dreary outside and will help you feel better.

CaptionBlue is not a bad color to start with as the color if a blue sky, but if that doesn't help, try yellow, green, or even red colored lights. Of course, you have to be able to see the colored light reflecting off the walls and any curtains, so it might not work as well if you don't have a good neutral background for the colored light to fall on. Christmas light strings might work too, if you outline your windows with them. Fake it till you make it!