Use interior paint to color the walls and ceiling of the bathroom in a Western-inspired color. Neutral tones like white, beige, cream or brown are very conducive to the theme.
Replace the bathroom rugs as well as any towel or washcloths with a shade slightly darker than the shade of your interior paint.
Hang horseshoes on the wall for a rustic touch. These can be mounted on a simple nail hammered into the wall. Hang a single shoe over the door or toilet, or create an arrangement with multiple shoes such as a line of four over the bathroom mirror.
Put a pair of spurs somewhere on display where they won't be in the way. Mount them on nails as well and choose to place them on the wall or on top of a cabinet or counter.
Add some nature to balance out the theme by placing a small potted cactus plant on a shelf or on top of the toilet.
Mount some Western-inspired artwork on any free wall space to tie the entire bathroom together. Western art can depict desert landscapes, cowboys or Western plant/animal still life.