Lay the mirror flat on the work surface covered with newspaper. Tape the glass part completely, using painter's tape to protect it from spills. Trim any painter's tape that overlaps onto the wood so the frame is completely exposed. If the mirror frame has a glossy finish, rub sandpaper over it to eliminate most of the gloss.
Distress the wood of the mirror frame so that it mimics old, worn wood. Press the head of a nail into the wood to make dents, tap a hammer against a nickel held against the wood and use a nail to create scratches. Use sandpaper to wear away a sharp corner or two. Be very careful as you distress the wood because if you use too much force, the mirror could break.
Paint the mirror frame in a color that matches your Western decor, such as muted blue, brick red or rich brown. Give the wooden frame two coats of paint. Allow it to dry completely, about 24 hours. Sand away some of the paint in spots with sandpaper, exposing the layers underneath.
Paint a dark brown stain onto an unpainted wood frame, if you do not want to paint the frame. Work the stain into any grooves, corners and natural imperfections of the wood. Rub the stain off immediately with an old rag. Allow the stain to dry for 24 hours. The dark stain will highlight the distress marks and the wood grain, giving it an antiqued, weathered look.
Add Western-themed embellishments to the mirror. Glue small metal shapes, like horses or stars, in the corners. Or, outline the wood frame with faux brass nail trim or leather braiding. Use glass etching cream to put the silhouette of a cowboy or a mountain range along the bottom of the glass with a stencil.