Get a Not-so-new Gown
Purchase your gown secondhand -- reducing fuels used in creating a new gown and lessening demand for overseas labor. One option is the Brides Against Breast Cancer campaign, run by MakingMemories.org, which hosts trunk shows across the country featuring donated gowns as well as new gowns and samples from designers and bridal salons. Proceeds fulfill wishes for terminal breast cancer patients.
Donate it to Charity
If you aren't planning on saving your dress as an heirloom, donate it to charity after the wedding. You can either give your gown to an organization like Making Memories, or sell it (be it through a consignment shop or online) and give the profits to the charity of your choice.
There's also the option of having your gown custom made with natural fabric (most likely silk or a silk blend) that you've purchased yourself. If you don't have a seamstress near you (or the cash to afford a gown that's completely custom), Conscious Clothing, based in Santa Fe, NM, creates wedding gowns that range from traditional to trendy, and feature all-natural fabrics (visit GetConscious.com).
Don't Forget the Maids
Ask them to donate their dresses to charities such as The Glass Slipper Project (GlassSlipperProject.org) or the Fairy Godmothers Inc. organization (FairyGodmothersInc.com), both of which work to put dresses in the hands of underprivileged teens who can't afford a prom dress. It's a much better fate for the dresses than sitting in the back of closets -- trust us.