Mouse, if you haven't done so already, get a complete physical. Lots of things are linked to depression, like hypothyroidism, diabetes, and PMS. Alcohol is a depressant, so if you drink it, you might want to try cutting down or stopping.
The stuff that Shallowford linked to is interesting, and to my experience, true. Do your best to eat a healthy, well-balanced diet (though I'll freely admit that this easier some days than others). Physical activity (something you enjoy, rather than something that feels like a chore) is also important. Get outside in the sun (if you still live in SoCal, that should be fairly easy, especially at this time of year, when it gets light before 5 AM and stays that way until past 8 PM).
Diet soda is all kinds of bad news -- for your mental health, for your bones, for your weight -- and should be limited or avoided altogether. Regular soda is slightly better, especially if it contains actual sugar instead of high-fructose corn syrup, but too much of that isn't good for you, either. Water is good for you, and tap water in this country is generally safe and reasonably cheap.
Also, chocolate can be a good thing.
