I haven't repainted my bathroom since I learned about this, but this is a must for painting a bathroom. Just glue a paint stick (free) to a paint pad and drape some plastic around your toilet and just repaint the area behind the tank and toilet with ease. I do use the paint pads for doing corners and edges, so I know they paint like a dream.
The only sure way to keep water from reaching the backer board is to waterproof all areas that may be exposed to water. That's easy with the new waterproofing coatings (RedGuard is one brand). If you're in doubt about potential water problems, coat the area with waterproofing. Follow the application instructions on the container. You can apply it with an inexpensive paint pad, which allows you to quickly spread a thick, even layer. You can also use a brush or roller.
DIY fingerprint paint pads and bugs Use handy shammy from dollar store. Cut in smaller pieces, get wet, wring out and put a small puddle of paint in the middle of each one. Rub the paint in with a finger.