Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
Mud slide/rock slide
Road sign barriers
Parking space lines
Strung rope or wire
Protected parks and designated areas
Border/Customs between countries
Great wall of China
*Barriers can also be economic, educational, physical health restraints, mental limitations (fears, phobias) and popularity (money, degrees, handicap, memberships, notoriety)
These are just a few examples of things one might associate with Borders/Barriers. Some are more powerful than others. A concrete barrier wall is a strong symbol, and likely will not require reinforcement. However, the presence of a baby gate in a home might not foreshadow a border/barrier on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.