If you will be placing the mirror where children play, use safety glass.
To prevent moisture damage, seal mirror edges by painting “edge seal” (a lacquerlike substance available at glass shops) around all sides.
Vertically mounted mirrors
Add a backing such as exterior-grade 3/4-inch plywood. Use mirror mastic, a strong waterproof adhesive that’s available at large hardware stores and glass shops, to mount the mirror to the plywood.
Give the mirror proper support.
Frame the mirror with wood, metal, plastic, or even tile.
Mirrors on the ground
Choose a level spot among plants.
Dig out 2 inches of soil and replace it with builder’s sand, packed firm.