Find some old doctor/nurse things (stethoscope, name tag, blood pressure cuff, doctor jacket, ect.) and put them on. After that get a black and white wig or black and white spray paint and put it on your head. Make sure to tousle your hair (wig) up to give the "been dead a while" look. Then you paint your face white or put baby powder/flour on it. Now you have a perfect "ghost doctor" effect.
Be sure to have a bunch of props laying around when your trick or treaters
Arrive like compressor stinks, a clipboard, a scale, a jar that looks like it has an organ in it, ect.
Thanks to Britt for this fun idea.