Symbolism
things that mean 'Arwen'
There are a few things that symbolically represent the Lady Evenstar.
the Evening Star
In Tolkien's world, the Evening Star was the light from the Silmaril bound to the brow of Eärendil -- Arwen's grandfather, as fate would have it -- as he took his great vessel to the skies every evening. Family connections aside, Arwen is known as the "Evenstar of her people" because she is the last great light of the race of Elves before they dwindle into darkness and obscurity. She is the last shining example of elfhood before the Age of Men begins.
The evening star is a symbol that is repeated many times, with many different aspects, throughout the course of the world's mythology and stories. Just like in Middle-earth, our evening star is also not technically a star, but rather the planet Venus, rising in the West very shortly after sundown. It is the brightest object in the night sky besides the Moon.
In western astrology, Venus is the planet that represents love and passion. Considering that in Arwen's life, love and the sacrifices made for it are a central theme, it is very fitting, despite there technically being no Venus in Arda.
The Evening Star, one must not forget, is also the selfsame light as the Morning Star. The morning star to Arwen's evening was her foremother Lúthien Tinúviel -- and it was commonly said that Arwen indeed held Lúthien's beauty. The first words Aragorn ever greeted her with were "Tinúviel, Tinúviel!", thinking -- perhaps while in a fog of adoration -- she was Lúthien brought back to life!
white jewels
There are two predominant white jewels in Tolkien's mythos -- diamonds, called adamant, and clear crystal. Arwen's Evenstar in the movie is made of white crystal, and in Book Six, she gives to Frodo as a parting gift a "white gem like a star upon a silver chain". In Book Two, she is at the feast wearing a silver lace cap, netted with white gems.
Diamonds traditionally symbolise eternity, because they are near-impossible to break down. This is a VERY good symbol for Arwen, for as she is an elf, she was meant to live forever. However, as we know, she does not -- she dies of grief shortly after Aragorn's death. Yet, note this: in the instance we see Arwen before Aragorn's ascension as King, she is wearing her silver lace cap netted with white gems...and after their marriage, she gives away, to Frodo, the white star-gem, and with it her passage to Valinor -- her ticket to eternity, more or less. Interesting, isn't it? Did Professor Tolkien plant this symbolism there intentionally, or was it just chance? In the movie, Arwen's Evenstar pendant was made of diamond, which she gives to Aragorn. This symbolises her giving to him not only her life, but her life and her eternal love. When it breaks, in Aragorn's dream in Return of the King, we see Arwen dying.
The ancient Romans believed that diamonds were splinters of fallen stars, which also fits Arwen -- should an not Evenstar be adorned with her natural element?
butterflies
Used very often in the movie, Arwen's apparel often incorporates butterflies (her brooch in the outfit she wore while riding out of Rivendell to the Havens is the most notable). Once you think about it, it's a very clever metaphor.
Butterflies symbolise transformation: they emerge from the safety of their cocoons into the sky as beautiful insects with glorious wings. Arwen can be seen as a butterfly in that respect -- she emerges from the safety of her Elven life and her father's protection into a whole new world, the world of Men, to spread her wings as Queen of Gondor.