According to The Phrase Finder, "something old" respresents continuity in the form of the couple's friends or family who will support them during their marriage. Items usually are given to the bride by a happily married woman, to pass the happiness on to the new bride. Often the "something old" will be a piece of family jewelry or other keepsake item. My husband's family has a diamond that has been passed down through generations and reset in each wife's engagement ring, so I had a special something old right off the bat!
No shortage of options here—but for the traditional symbolism, Georgia Girl's page on wedding traditions says that the wedding gown is often chosen as "something new," meant to represent good fortune, success and good hopes for a bright future in a new life. It makes sense that you'd want to step out to your new life in your new dress!
The symbolism behind "something borrowed" is similar to "something old," except that the bride would have to return the valued item to ensure good luck, according to The Phrase Finder. Georgia Girl agrees and restresses the value of the borrowed item coming from a happily married woman.
This is actually the oldest part of the traditions and has lots of meaning behind it. Ancient Romans believed that blue was the color of love, modesty, and fidelity, according to Ask Yahoo. It is also well known that the Christian Virgin Mary is most often depicted wearing blue, and the color has been associated with purity since biblical times. All three of the aforementioned sites mention that in ancient Israel, brides would wear a blue ribbon in their hair to represent fidelity, and often both the bride and groom would wear a blue band on the bottom of their wedding attire. Before the late 19th century, blue was a popular color for wedding gowns. (I LOVE this idea, see the blue gown I found!)
A Silver Sixpence in Your Shoe
A quick lesson for the non-British among us: Wikipedia says a sixpence is a coin worth 1/40th of a British pound, made of sterling silver until 1920. The first sixpences were struck in 1551 and continued until the acceptance of decimalisation in 1967. Ask Yahoo says that the shoe custom may date back to a Scottish custom of the groom putting a silver coin under his foot for good luck. In any case, the sixpence represents financial security and wealth of both money and children. Some brides substitute a penny or a dime in their shoe (left shoe for best results). Andy's grandfather collected foreign currency and I was lucky enough to find an actual silver sixpence!
This whole poem is the most purely symbolic tradition in wedding lore, and can be taken as seriously or as lightheartedly as you want. I had the old diamond, new dress, borrowed my mom's pearls, wore blue underwear, and had the sixpence in my shoe because although I'm not superstitious, my wedding wasn't the time to push it!