Adhesive spray or painter's tape holds the stencil in place. Most of the time, tape is the easiest way to adhere a stencil. It's also useful for masking off areas of the stencil you don't want to paint yet. Low-adhesive tape works best, because it adheres well yet won't mar the surface or pull off the paint when it's removed. Keep plenty of low-adhesive tape on hand for every stenciling project.
Although re-positionable adhesive spray is a little more expensive, messier, and smellier than tape, certain situations call for its use. On a smooth, slick surface, for instance, spray adhesive can be helpful in keeping paint from seeping under the edges of the stencil. Spray adhesive is also ideal for stenciling on tricky or hard-to-reach areas such as ceilings or curved surfaces.