A quick, easy way to add some interest to plain walls is to use a simple one overlay design.
This design (#903 Orient Express) by Royal Design Studio, is done using a drop shadow technique. The stencil is first done using a soft black. Once dry, the stencil is repositioned, about 1/4″ up and shifted to the right a bit. The design was then stenciled again, using a metallic brass paint.
I like to spray the back of my stencil with repositionable adhesive, when doing this type of stenciling. It helps to prevent the paint from bleeding under the stencil. Also, remember to always use a very dry stencil brush, off loading your paint on a paper towel before each use.
When you use a single design, single overlay stencil, you have the option to repeat the design however you wish on a wall, such as using it as a border or a random repeat pattern.
This design, another Royal Design Studio stencil, was done using Copper Metallic Palette Art. I believe this product is now marketed under the Faux Effects line and called Palette Deco. Any plaster-like material will work for this method. Once dry, the stencil was repositioned and a black craft paint was sponged over the copper to give it a hammered copper look.