Junarc, the main thing is keep your mind active. Depression is all in the mind. So if you are depressed, do something creative - do that home project you always wanted to do, read the book you have not finished reading, grab your camera to take pictures of sceneries along the corniche, clean up you room or arrange your closet - anything that will keep your mind thinking you are lonely and depressed.
As greeker says, put on a happy face.

And here's something from Nat King Cole:

Smile though your heart is aching
Smile even though its breaking
When there are clouds in the sky, youll get by
If you smile through your fear and sorrow
Smile and maybe tomorrow
Youll see the sun come shining through for you

Light up your face with gladness
Hide every trace of sadness
Although a tear may be ever so near
Thats the time you must keep on trying
Smile, whats the use of crying?
Youll find that life is still worthwhile
If you just smile