I don't know about 'the best way.' I expect that lies in the realm of the Psychology experts. However, I've met with a few people who were depressed, even diagnosed as clinically depressed, and feel I've been able to help bring them through a lot of their problems and into a more positive way of looking at their lives.
First thing is friendship. Be a friend who is willing to meet with them over a cup of coffee - or even a day out somewhere. Do a lot of listening. Keep the conversation rolling but be aware if they just want you to be quiet. Let them lead the conversation and take a genuine interest.
Write them letters. Not just emails or cards, but genuine, indepth letters about your own feelings about life. They will then feel that they need to say their piece in writing to you, too. This will help them clarify a lot of things in their minds.
Keep it up. You don't have to do it a lot, but do it so that they feel they do have a real friend in you.
Hope this has helped.