Every day we interact with objects, places and sensations that affect the way we think and feel. This can be used to the writer's advantage by planting symbols in the reader's path to reinforce a specific message, feeling or idea.
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?
A lone tree on a hill
An animal driven away from the herd/family group
A single bird flying without a flock (geese, for example)
A lone wolf
A falcon or eagle's nest at the top of a tree
Contamination units in hospital
Victims of abuse
Victims of alcoholism or drug abuse
'Corner Kids' (kids in schools who are not accepted by others)
Solitary confinement (prison)
Adrift on a raft
Alone on a boat
A teen with headphones in
Fishing shelter on a lake
Being snowed in
These are just a few examples of things one might associate with Isolation. Some are more powerful than others. A snow-shrouded cabin in the woods is a strong symbol, and likely will not require reinforcement. However, a lone wolf may not foreshadow isolation on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.
Closely related to Isolation: Alienation.