Light tableA light table is a very useful piece of equipment. You can use it for various things:
Masking Fluid / Masking TapeSometimes a picture requires a part to be left blank (for coloring later), while you want to paint the background around. It can be very hard to paint "around" a character, so you can use masking fluid instead: a liquid (white or colorless) with rubber/latex pigmentation for masking out areas of work needing protection (when color is applied in broad washes). When it's dry, it feels like a thin layer of rubber. However, it shouldn't stay on the paper longer as neccessary (for best results remove within 24 hours). The brush must be cleaned immediately after use (otherwise your brush will be permanently damaged).
For work like airbrush, there is also a special masking tape/foil that you can put on your picture and cut out the area to be protected.
Artist TapeThere is a special artist tape (usually white), which almost feels like paper (it can be easily ripped by hand, yet it's pretty solid). It is smooth, acid free and repositionable (no traces or damages when removed).
You can use this tape for holding your artwork to drawing boards, so it won't shift while painting. Also, when drawing with water colors, it will help keeping the paper in shape (so it won't ripple too much). Don't remove it while the paper is still wet or the paper might tear.