Since I have battled depression off and on, I know a thing or two about it. It rare that anyone who has dealed with depression to fully get over it.  The best thing you can do is to manage it and do things that are not going to bring it on. 

Simply said, its a chemical balance. Environment and other factors can play a role in this. 

• Persistant Sadness
• feeling worthless
• Difficulty in concentrating
• Poor Memory
• Insomnia or oversleeping
• weight gain or loss
• anxiety or irritability 
• thoughts of death
• headaches,stomachaches and digestive problems 

First off, be understanding of the person whom is battling depression. Don't expect the person to just snap out of it because it is not going to happen. What you can do is be understanding and try to help them through it.

Some things you can do to help with depression is to do that do things that make you happy. I have found that when I am having one of those problems, its best to actually write or play in photoshop as it does help for the mood. What I do may be different than someone else, but the whole idea is that if you battle depression is to do something that helps divert the mood.