We’re sometimes taught that authors "plant" symbol in their work. In other words, they consciously decide to put a given symbol or metaphor into their fiction. But symbol and metaphor most often don't work like that as we're writing, do they? I usually don't know why a symbolic element is important to the novel until the revision process, and sometimes I don't even recognize it until much later, after the book has been published and readers start telling me what they see in the novel. I just know that the element must be there; I know it intuitively.
 
Still, I believe writers need to explore symbol and myth from all world cultures and their own, for inspiration. To that end, while I did my creative writing degree at university, I also studied the classics, Roman and Greek myths, and comparative religion, and I continue to read anything I can get my hands on that explores symbol and myth.
 
But, of course, we must be cautious of plucking symbol from a book and planting it in our writing. You don't want to bang your reader over the head, effectively saying, "Look, a symbol! Get it?" A subtle hand is required.
 
Or, alternatively, you can take your symbol and run with it. Is the guy in your novel feeling like an outsider? An alien? Why not make him an alien? Turn him green, let him grow an extra arm and two more eyes and have him walk around his high school like that. In my middle school novel, Iggy’s World, Iggy, who loves bugs but feels like an outsider, finally lands an acting gig wearing a bug costume in his father’s web series. He’s still an outsider, but has found a place for himself, as the thing he loves (a bug!). Don't be afraid to let your symbols run away on you from time to time.

In the end, working with symbol and metaphor isn’t about planting elements, but about discovering what's already there. Once I have a first rough draft done, I begin to comb through my writing looking for what I call "bells:" repetitive images and symbols that arise naturally from the situation and setting I’m writing about, and that contribute to the theme of the project. I highlight these repeating elements and map them out with mind-mapping (spider drawings) like the one above to see where they occur on my plotline, and then brainstorm with those elements to develop them further. In the end, my goal is to have a hammer somewhere in the novel (usually towards the end) that gets all those bells ringing, that allows the theme to resonate throughout the story or novel.

If you own a copy of Jack Hodgins A Passion for Narrative, check out his excellent chapter on the topic titled, "Making connections: Metaphors, Symbols, and Allusions." In the meantime, here's a primer on symbolism that offers examples.