I don't think I have anything new to say about writing and depression, but I'm struggling with it right now and trying to reboot myself out of it, so I thought I'd talk about that.
For anyone who's new here: I have bipolar I, which means that I spend significant periods of time depressed. I also cycle very quickly sometimes, so I can go from elated to suicidal within a single day or the course of hours. Needless to say, besides sucking in its own right, it makes writing, which I think of as a somewhat neurosis-inducing career , an additional challenge.
 I am pretty sure there are non-neurotic writers out there! But I am literally, professionally diagnosed crazy, and I have spent time in the psych ward for suicide attempts, so...
When writing gets hard, it comes down to routines. Writing is easy when it's a fire in the mind and the words blaze to be let down on paper (or typed into the computer, or whatever--I write both longhand and on a computer depending on my mood or the particular project). But inspiration is completely unreliable, especially when depression comes calling.
My routine goes something like this. Note that I don't claim that this works for everyone! Just this is what I do, and it more or less works for me. Sometimes better than others.
1. Get out of bed. Sometimes this is the hardest step.
2. Get food into myself. I have this rule that no writing happens until I have eaten something, even an oatmeal packet. Bodies are weird (or anyway, mine is! maybe yours is perfectly fine :p). If my blood sugar drops, I turn into a depressed suicidal wreck. I find this completely maddening considering that I'm overweight so you'd think that I could survive for a couple extra hours off fat reserves, but nope! Not so lucky. So I try to remember to eat at intervals. Even so, there's this period in the late afternoon/early evening where I usually have to take a break from writing no matter when I started because my blood sugar is too low for me to concentrate. (This is usually because I'm trying to time dinner to be convenient for my husband and daughter. If it were just me, I would eat smaller meals every four hours and that might work better.)
3. Get exercise. Sometimes I skip this, but I read somewhere that you should try to do the most important things first in your daily routine. I figure exercise is more essential than writing, or anyway, it should be higher priority. Also, I sort of cheat in that right now I'm mostly doing the world's wimpiest exercise biking, on a bike that has a built-in desk, and I use that time either to do reading (right now I'm beta reading for someone, for instance), or write fanfic. I could even use that time to do work-writing rather than fanfic-writing. It all depends.
4. Get shower. Because I am so wimpy, even wimpy exercise-biking leaves me drenched in sweat.
5. Make tea. I allow myself one cup of caffeinated tea a day. Right now that's a Republic of Tea black tea flavored with almond, which honestly I don't like all that much--I tried it out of curiosity and discovered the almond flavor didn't agree with me. So when the tin runs out I'll switch it for some other black tea. After that runs out, I start making herbal teas instead. Too much caffeine can trigger mania or hypomania, or just generally screw with my sleep (and screwing with sleep can mess with bipolar cycling--it's a whole Thing), so I try to not to overdo it.
6. Settle in to write. I turn on iTunes, set the whole thing to Shuffle, and attempt to write at least one sentence/song. Most songs are pop/rock songs of 3-4 minutes. This is not a recipe for blazing fast writing. What it is, is conditioning. My brain gets the idea that every time we switch to a new song, I should get back in gear and get writing. My philosophy is that slow and steady wins the race. I don't produce words particularly fast--I know there are fast writers out there, but I'm never going to be one of them. But I do believe that accumulating words a little at a time consistently will also work.
One of my problems is low morale, and a related problem is being intimidated by high goals. So I set low goals. One sentence during a song of that length is eminently doable. In fact, spurred by one thought, I usually end up writing more than one sentence. And that's good! Likewise, when I am at my most depressed--when I can barely string two thoughts together, or when I feel like everything I have ever written is completely worthless, I set my goals very low. As in 250 words/day low. These days I have novels to write so I can't do that forever, but even 250 words/day is better, in terms of sustaining momentum, than 0 words/day. It's simple mathematics. If you write 250 words/day, you can eventually write a novel, even if it takes you a while. Whereas with 0 words/day? You'll never get there.
This is not to say that you should never take a break! I have 0-word days. Weekends are usually dead time because I have family obligations. Sometimes the depression is just too much to deal with. But there is a difference between occasionally taking a break, and never writing. The latter is what I seek to avoid.
In the meantime: what helps you when you're dealing with doubt or depression? Tell me one thing you like about your own writing, if you like. :)