Flower essences can be extremly helpful for depression, however it's important to know what type of depression you have to get the most benefit from essences.  If you think you have depression then you probably do, but it can sneak up without you even knowing you have it.  It's not always as obvious as crying all the time.  Malaise, irritability, sleeping too much or not enough, loss of enthusiasm can all point to depression.  I'm not saying that if you have any of those, that you are depressed, but don't rule it out.  I'll link you to a quiz at the bottom. 

If you know you're depressed, and you've been down for a long time even though things in your life are going ok, then your depression is most likely a chemical or electrical imbalance.  Mustard flower essence can be helpful here, but you will probably need to work on this with either medication and/or nutritional therapies.  An excellent book on brain balancing is The Edge Effect by Eric Braverman.  It has a test you can take to see what neurotransmitters you're dominant and deficient in and what course of action to take based on your results.  You may need to tweak your diet, take some supplements or get your doctor involved.  In either case it tells you exactly what to do for your individual scores.  This goes way beyond depression, my family and I have had good results following his recommendations for a myriad of different issues.

If your depression improves significantly when good things happen, then you have situational depression and flower essences can definitely help you.  You might think it's normal to be down when things aren't going your way, but I'll tell you it's not.  I have a deep unshakable joy even though I might be upset or sad over a death or some other current event. I am excited about life and even when I'm down, I'm feeling joy and gratitude and optimism.  It's normal to react to things, I think there's a difference between happy and joyful.  Happy is circumstantial, at least to me.

If you get a little better when life is going good but still feel depressed, then you have a mixture of situational and clinical depression and you need to hit this from all angles.  The good news is, whatever modalities you try are likely to help you to some degree.

Here are some essences to help with depression.

Borage is useful for grief, depression, discouragement, anything that produces that "heavy-hearted" feeling or prevents you from having the confidence to face a tough period in your life.  Borage is uplifting and expanding.

Bluebell helps those who dwell in the negative, who are grumpy, moody, and cannot cycle out of cynicism. Bluebell helps give you a positive outlook on life.

Honeysuckle is for letting go of the past, whether it be too good and you no longer like where you are in time, or to let go of an experience that has affected you negatively.

Horseradish helps those who feel stuck and unable to make changes, a victim of fate or under others control, horseradish deals with those feelings of fear, depression and powerlessness.
Mustard helps with unexplained depression that hits you from out of nowhere. For more info go here.
Onionis good for grief, helps with unexpressed emotion, or for those who cry all the time
Saint Johns Wort is all about the light.  It brings about illumination of all kinds, from shining light into your soul, or for seasonal affective disorder, or for sensitivity to light or other environmental stresses.  Gives courage, strength and spiritual awareness.  Useful for those who need to become more tolerant of their surroundings. For fear of the dark.
Sweet Chestnut For those who do not believe they have reached their threshold of pain and can't bear anymore, no way out.  Sweet Chestnut will get you through in one piece, and give you back your faith.  If you are suicidal get help and mix Sweet Chestnut with Bach's Cherry Plum essence
Thimbleberry brings joy despite circumstances
Wild Rose helps those who are overly accepting of a cruddy lot in life, they don't even try to work towards betterment, they are resigned to their "lot in life."  This occurs frequently in long illnesses or "uncurable" illnesses.
Zinnia will make you laugh. Zinnia is for people who take themselves too seriously, for repressed inner children and workaholics. Zinnia helps bring humor and fun to your daily responsibilities.