"I should reach the frontier in another five weeks."

Cultivating symbols in your own work should come after you have already fleshed out a story's outline, or have written a treatment, or better yet, after a first draft. If you try to work symbols in too soon, they will bog you down, and may even take over the narrative. They're greedy that way.

Rather, the proper approach is to first have a deep understanding of your material, your characters, your point, and then a really good grasp of your theme. Theme is where symbols live and thrive, and where they best serve your story. Once you know what you are trying to say, you will already begin to see symbols suggest themselves.
Some questions to ask are:

• What symbolic relationships exist between my characters?
  Father-Son: Luke Skywalker and Darth Vader in "Empire Strikes Back". The main theme of their relationship is that of a boy's desire to kill his father (how shockingly Freudian!), and holds true even before Luke knows that Vader is his parent. This is a common relationship for antagonist and protagonist to have, the antagonist having the position of authority figure who must be overcome or beaten, thus exceeded, by their "offspring."
  Teacher-Pupil - Jacob and his chiropractor Louis in "Jacob's Ladder". Jacob's chiropractor literally adjusts Jacob's body, therefore "adjusts" his attitude.
• What symbolic situation is my character in?
  A wealth of goods, but a poverty of life - Michael Douglas in "The Game". The course of the story takes him from one extreme, that of a rich man with a cold life, to the other, that of (seemingly) total poverty, but living on the edge. This same theme is played out in "The Edge". The theme of both thus becomes rebirth through confronting death.
  The desire for action - Lawrence in "Lawrence of Arabia." He has a penchant for letting matches burn down until they scorch his flesh. It symbolizes both his tolerance for physical extremes and his need for extreme stimulation.
• What symbolic traits does my character have?
  Nosy - Jake Gittes in "Chinatown." He has his nose slit open for prying.
  Passionate - Ada in "The Piano." Her piano is a mirror of herself, specifically her "voice" and her feelings. In a moment of marital infidelity, she sends a piano key to her lover with a note inscribed upon it. Her husband's revenge is an attempt to silence her passionate "voice," and he chops off one of her fingers, making her the symbolic mirror of the damaged piano.
  Truthful - Graham in "sex, lies, & videotape." Previously a pathological liar, Graham now strictly tells the truth. In perfect symbolization of the trust engendered by truth, he has very few keys, which are a symbol of distrust.
• What is my theme? What is the story really about?
  Self-determination - "Thelma & Louise." The two women seek the right to determine the course of their own life, a course that is denied them as women in a man's world. In killing themselves, an ironic symbolic rebirth, they finally gain control of their destinies.
  Humanity - "Blade Runner." The replicants are constantly equated with children (unformed humans) and animals (non-humans). Child symbols: their appreciation of toys, play, and the photos they collect in order to hold on to their brief past. Animal symbols: the glowing red-eyed glances of the replicant owl is mirrored in several of the replicant humanoids. Zhora is covered with scales after her snake dance as if she were the snake, then tries to strangle Deckard with his tie - as a snake would constrict its prey. Roy howls like a wolf as he chases Deckard.
  Deckard is symbolically equated with a replicant, in that he begins as an emotional cripple, has a large collection of photos, constantly interfaces his world via machines, and is also a cold-blooded killer. He finally finds both feeling and humanity, ironically because of the replicant Rachael.

Is your head spinning yet? These points will give you a lot to think about, but don't be overwhelmed. Rather, begin to look for the use of symbols in the films you watch and the stories you read in order to see what they are and how they work. Once you have gained a working knowledge, you may find that you are then able to think of the kinds of symbols your own work could contain.

Play with them, revel in them, but use them wisely. When properly utilized, symbols are a powerful elements of thought-provoking narrative.

Related Links:

The "Alien" Screenplay
By Dan O'Bannon, Walter Hill (uncredited), David Giler (uncredited), and Ronald Shusett (story). Get your own free copy of this work online. The script is a masterpiece of understatement, the screenplay equivalent of a hard-boiled detective novel in that its descriptive language is so stripped down as to read like a beat poem.

Giger.com
The official H.R. Giger Website, with lots of disturbing images and merchandise for you to browse or purchase. Yes, true evil can be charged on account!

The Three Laws of Robotics

1. A robot may not injure a human being, or, through inaction, allow a human being to come to harm.
2. A robot must obey the orders given to it by human beings except where such orders would conflict with the first law.
3. A robot must protect its own existence as long as such protection does not conflict with the first or second laws.

As proposed by Isaac Asimov in "I, Robot."