We had a shower curtain destined for the trash, but I just knew I could come up with something to use it for. So begins the idea for the black light reactive disappearing ghost. This is more a proof of concept project, I hope someone with more time and artistic talent will take the seed of this project and grow it into something fabulous. (Please post pics and/or links if you do - I'll give you a "That's what I had in mind" patch)

Supplies:

Clear shower curtain or other plastic sheeting - I don't think it matters how thick or thin it is, as long as it holds up to your display method.
Liquid laundry detergent - the brands I have tried are All, Tide & Whisk. You may want to test yours first to be sure is has the proper UV reactive additives.
Rag, sponge, or sponge paint brush - the tool will depend on how precise you want your image.
Blacklight bulb

Step 1: Paint Your Ghost

Lay out your plastic sheeting on a flat work surface. You don't need to do much to protect the area, as the soap is easy to clean up - making it a perfect decoration for kids to help assemble.

I poured a bit of soap into an empty cream cheese container; just the right size for my brush, easy to dump the leftover into my next load of wash.

Paint on your ghostly apparition. I would recommend having the black light on during this process. I painted my ghost under normal light and really had no idea what shape I was getting. The soap, though it appears blue in the container, is colorless when sponged on.

Step 2: Hang 'em High

Find a spot you wish to haunt and hang him up. In the dark you see nothing, under normal light you may or may not see the plastic/vinyl. When a black light shines, the ghost leaps to life. It is tough to get good shots of black light projects, but I'm posting a few to give you the idea.

The video shows my ghost indoors & out, so you can see it in different conditions.
For the outdoor shots, I put my black light bulb into my motion sensor lights - when trick-or-treaters cross it's path, the ghost appears.
For the indoor shots, I just had the bulb in a standard workshop light and shone it on the surface.

Step 3: Variations

A full width shower curtain could easily become a whole gang of ghosts rising from your grave yard.
Attached across the corner of your porch (perhaps with a pressure sensor to turn on your black light) it would be a spooky greeting for the kiddies.
Someone with artistic talent could do more faithful representations of actual humans - perhaps your own family - to add some genuine creep factor.
For ease of completion, you could have all the kids make hand prints on the plastic, complete with drips, to look like someone is trying to escape.
Because of the ease of clean up, you could probably paint the soap directly onto your windows and have ghosts appear in the window. Perhaps directly onto your outside walls, as well - or as ghost hands or face that suddenly appears right on your tombstones.

I hope someone will make this as an actual decoration - I would love to see it executed more carefully (something I intend to do next year)

Thanks to tqwerty, whose nail polish project introduced me to the wonders of laundry detergent as craft supply