I suggest talking to a consoler or a psychiatrist. It's good in all if you can deal with it yourself , but it's better to see a professional so you can get to the root of the problem and see why it is your depressed and work on fixing it.
Spill it all out to there friends and maybe see if they can tell you positive things are something. But only have friends help you if your not self-destructive. Why? Because only a good friend can get to know you enough to figure out the problem, (but if you are immensely depressed it will not work,) and they know you best and they can easily help.
difficult to say because it usually takes me over. it's chronic and acute for me. I just try to get through it and tell myself things that are helpful and usually true. You gotta realize when you're depressed that it's all a bunch of bull and that you are worth it :) a book i would recommend you checking out is Feeling Good. I bought it on Amazon. it's an easy read and tells you all the helpful things you need to know about depression ;)
I try and think about 4 positive things in my life or 3 good things that Happened in the last week and focus on them. When this doesn't work I normally go to my bestfriend and just talk about everything. And a good cry always helps :)