Great post. If you do not mind, I'd like to add a couple of recipes since I just finished the repaint of a Mata Hari cabinet, using Pimball Pimp stencils.
1) Go slowly when removing the backing paper holding the stencils. As Timab very justly indicated, it is easy to forget a small detail and ruin the stencils, later on trying to fix it. I remove the backing paper very slowly, a couple of inches at a time, checking continually for pieces of stencils that could have been left stuck to the backing.
2) Paint a very light coat around your stencils. Paint horizontally as much as possible. Most of the second and third colors on classic pinballs, are dark colors and do not need a thick coat to show up. Original cabinets were painted with just a haze of spraying on less than airtight metallic stencils. The lighter the coat, the easier it will be to remove the stencils. Do not wait for the paint to dry before removing the stencils.
3) Have cutter (X-acto), scissors in your pockets, ready to be grabbed and trash-can near by. I also use a fine pliers to pick up pieces of stencils where my fingers could not fit or risk making a mess. After removing partially the stencils, make a ball with them and keep them in the palm of your hand. When panicking, just cut the stencils, and trash the piece that has been removed. Breathe and go back with your pliers, fishing for more stencils to remove.
4) When a line is not perfect or a piece of paint falls back on the edge, I use a cotton swab with Naphta. Place it very carefully near the blob. The cotton fibers will gently suck away the extra paint, leaving a perfect delineation. I saved my Mata Hari cabinet many times with that little trick, especially around the hair and face.
Pimball Pimp stencils are first class but require a lot of care to be used with success.