stay in a state of joy.... ...It is also chemical, but I personally believe certain ways of thinking can cause imbalances.
This is great advice. I have struggled with my depression for more than 10 years. OCD for almost 2. Honestly, pregnancy is making it easier for me to be more proactive about my thoughts and activites because my mood directly affects my LO. For example, if a "sad" song/commercial comes on flip it immediately, or if a book/article goes somewhere "sad" put it down and do something else. Don't go to scary/dramatic movies if you're the type to dwell.
At dinner or in a journal talk about the worst part of your day and then the best part. Remember that even the worst days are only 24 hours long, then it's a clean slate.
Give yourself a treat everyday, a 10 minute nap, ice cream, dance party (close your door and rock out to your favorites), permission to skip a chore, etc.
The thing that has always helped me the most is knowing when to ask for help and what to ask for. I know that if I tell my husband, "I need a hug", that he is going to understand, not ask questions, and hug me until I tell him, "enough". Same goes for, "I need to talk" and "I need left alone (he'll give me an hour then check on me)". When I am having good days we'll talk about my depression and he'll ask what he can do.