My personal depression was most likely chemically induced (hormones). I had tended toward depression in my younger days, but didn't really recognize it as such. Just thought I might be moody and didn't always feel that I "fit". It became much more, though. My thoughts:
1. Not all depression is of a spiritual nature. Some is situational (death, trauma, poor health, loneliness, etc.), and some is caused by a chemical or hormonal imbalance. Various spiritual people in the Bible suffered from depression, so you're not "sinning" because you're depressed!
2. It isn't wrong to get medical help for depression. We've been taught so strongly against medical intervention for mental problems in Christians that we're ashamed to admit there's a problem at all. Yet, we don't struggle with that dilemma if we have diabetes or cancer. If you've tried vitamins, herbs, etc. and there's little or no improvement, you may want to seek medical help. I , personally, had to go on Lexapro for a couple years, but no longer have to take it. It didn't completely get rid of the depression, but it made the lows not quite so low. Unfortunately, I felt it made the highs not quite so high, as well.
3. Find the trigger. Many times, bouts of depression are cyclic or triggered. It can be as simple as a person's being put in an uncomfortable situation, or even the anniversary of a traumatic event. I still find the holidays difficult to cope with. I'm not especially comfortable in crowds - and that includes family gatherings. While I'm much better now, it was nearly impossible for me to deal with the "crowds" when I was ill. Try to determine if your lowest times are being triggered by a specific situation you're being put in (school, Sunday School class, family gatherings, a particular friendship).
4. Your thought life is important. While the "pull yourself up by your own bootstraps" becomes irrelevant in extreme depression (it's impossible to think that pro-active when you're looking up to see the bottom), I found it helpful to listen to Hosanna or Integrity praise and worship music. I prefer silence, but, when I didn't have something going in the background, my thoughts would automatically wander to negative or non-affirming things. I would find myself rehearsing injustices over and over again. I would discourage music with high tempo or strong rhythm. Those things minister to the physical but not the mental. Music in major keys (as opposed to minors) with even tempo and Biblical lyrics is best. If you're not into praise and worship, at least listen to something soft and soothing.
5. Find some key scriptures to memorize about your specific situation. Tape them to the mirror in your bathroom, write them on index cards and carry them with you. I would quote (recite?) "For God hath not given us the spirit of fear but of power and love and a sound mind" over and over and over again. That may not be the verse that speaks to your loved one, but there's bound to be one special verse for him/her.
6. Recovery takes time. Depression doesn't just happen overnight, and recovery doesn't come that quickly either. If it's necessary to get on medication, it will STILL take time and effort on your part to completely overcome this. You may not be suffering from extreme depression, but if you don't find the trigger and learn to deal with it at its beginning, I can tell you from personal experience that it worsens.
7. Seek out friends and family who understand what you're going through and are willing to love you through it. Some people aren't comfortable being around a person who's depressed, and, while that stinks, you have to learn that's not YOUR problem. Avoid people who "preach" to you. My personal feeling is that no one who hasn't suffered from depression personally should actively counsel a person who's suffering from the problem. Unless you're exceptionally insightful and compassionate, you can speak more harm and discouragement than good. Sometimes the best thing you can do is listen.
I know this isn't an all-inclusive list, but, for those of you who have expressed interest, maybe it will give you a few guidelines.