I chose to completely black out the background so that I wasn't visible at all behind the lights. If you want to see some of the person behind the lights, you can open up the aperture a bit, or try some side lighting on the person, shielded so that the light doesn't reach the camera directly. The light will need to hit the person only - otherwise you lose the impact of your "paint".
There are tons of resources out there to inspire and educate you on light painting. One of the coolest is this murder-and-mayhem video. They have a great BTS clip posted as well.
Canon 5D Mark II
65mm - ISO 200
30s - f/32