The five elements commonly used in Wiccan practice are: Earth, Air, Fire, Water, and Spirit.
There are several ways to represent the elements. One of the simplest of these methods is by use of color:
It's important to note that while the colors above are the 'traditional' colors used to represent the elements, they are meaningless if they don't make sense to you. In my personal practice, I see 'air' as white, and have always used white to represent it. Spirit for me is always lavender. A close friend of mine saw air as light blue (thinking of the sky) and fire as orange instead of red. The important thing is to discover what makes sense and works best for your personal practice.
Another way to symbolize the elements is with altar tools:
Earth: Pentacle (this is usually a pentacle carved or painted onto a flat, circular piece of wood or stone)
Air: Athame or Sword*
Water: Cup or Chalice
Spirit: There is no formalized tool to represent spirit
*There is quite a lot of dissent as to which tool corresponds to which element. Some traditions use the Athame to represent fire, and some to represent air. Some use the wand to represent fire, and others to represent air. The way they are listed here is a reflection of my personal practice as well as the way the tools are associated in classic Tarot.
Some people represent the elements by items that include that element itself. Here are some examples:
Earth: stones, pieces of wood, pine cones, salt, bowls of dirt
Air: incense burners, censors, sage bundles used for smudging
Fire: lava rock, candles, matches
Water: bowls or cups of water
Spirit: personal choice
For written work or art, there are still other symbols used such as Alchemical Symbols (these symbols were originally created as part of an alchemical system of identifying elements):
Earth: A point-down triangle with a horizontal line near the widest part of it.
Air: A point-up triangle with a horizontal line near the widest part of it.
Fire: A point-up triangle
Water: A point-down triangle
Spirit: A point-up triangle overlaying a point-down triangle creating a hexagram (the Star of David is identical to this symbol).*
*While this symbol is commonly used in this set, Spirit was not one of the elements in an alchemy sense.
Also among the commonly used, are the Tattwa Symbols (these symbols originated in India but are commonly used in western magical practice):
Water: Crescent Moon with points up
Spirit: Vertical Ellipsis (an oval that is pointed at the top and bottom)
There are many ways to symbolize the elements in your personal practice. Use the methods that resonate the most with you.