For Halloween this year, I went as a robot, wearing a silver dress with a slowly pulsing LED heart glowing visibly under the fabric.
The LED is a one watt white LED, which we’re running at about 50 mA. It has a wide viewing angle, and the star-shaped mount lies conveniently flat. The LED is wired up to the PCB with a pair of twisted magnet wires. Magnet wire is flexible and thin, which makes it hardly noticeable under clothing. It is controlled by ATtiny2313 (running the code from our Mac sleep light pumpkin project) and powered by three AAA batteries. The PCB corners were rounded off so it wouldn’t be stabby.
The dress was fully lined, which made it very convenient for mounting electronics. I pinned a makeshift pocket onto the liner, and tucked the battery holder and PCB in the pocket. I could feel the battery holder switch and turn it on and off through the fabric.
The LED was taped to the dress liner with medical tape to hold it in place. An extra piece or two of tape held the wires to make sure there was appropriate slack for movement. (A note on tape: use the good stuff. The cheap paper tape in the off-brand first aid kit only stuck to itself and the magnet wire. 3M plastic medical tape worked great and came off easily.) This makes it easy to disassemble after Halloween.