All really excellent advice.
Now, as a not doctor, I will remind everyone that not all low mood can be fixed with fresh air, sunshine & the endorphins from sex & chocolate. If it lasts longer than 6 weeks, you aren't just dealing with the blahs. If you're thinking about hurting yourself, or having thoughts of worthlessness & hopelessness, the best SAD light in the world won't help. Talk to someone. Start with a family friend. Then a family doctor. There should be no stigma attached to mood disorders: there is no shame in asking for help. Check out sicknotweak.org, founded by Michael Landsberg @heylandsberg on Twitter. It's OK to be not OK. Finding the right medication can be tough, but it can also be a Godsend.