Change your look abruptly, if you are wearing bright colors, change them to dark and dull colors. Mess up your hair a bit and stop beautifying your face with make-up, or you can wear only heavy eyeliner or mascara. No bright, fruit-scented lip-gloss. In essence you will want to stop taking care of yourself so that the mind feels that you are sad or upset and this might trigger you to look depressed. By neglecting your daily routine of putting on makeup and taking care of yourself, the mind will automatically sense that something is wrong and will enhance your feelings of being sad. This feeling of sadness will eventually make you start to look seriously depressed. You can also dress sloppy as if you do not care what you look like. Do not iron your clothes and wear the same things over and over. This will make you look sad and depressed.
Try to change the manner you regularly use to talk. Stop talking with a voice and words as fluently or confidently as a happy person does. Speak in low voice and try to act that it is hard for you to talk. Be ironic and a bit nasty, like a person in a bad mood. The whole idea is to give the impression that you are feeling low and depressed. You will want to basically convince yourself that are feeling sad and this will project a definite sense of being depressed that can be visually noticeable.
Continue to spend time with your friends, but half of the time, stay alone. Sit down or bend from a vertical position towards wall and make a thoughtful look on your face. Make it look like you are perpetually sad and lonely. This will add to the overall drama of looking like you are depressed and down in the dumps on an emotional level.