You could pursue a so-called utility patent for any functional or structural aspects of the dress, or if you merely want to protect the ornamental appearance of the design, you could pursue a design patent. The following Google search indicates that there have been many successful design patents for dresses...
Design patents cover the ornamental design for an object having practical utility. Utility patents protect the functionality of a given item. Clothing is usually not protectible by either kind of patent. Clothing isn't copyrightable, either, because copyrights are for expressions of ideas. However, what clothes designers can do is trademark their brand, so as to build up and become known as the source of the clothing. You should sit down with an IP lawyer to fully disclose your business model and get advice on how to organize and protect your business.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Unlike my colleague, I think that it's quite difficult to be granted a design patent on a dress.
The patents cited in his link are all about 60 years old and have long since expired -- and are now prior art, reducing the universe of novel and non-obvious dress designs capable of being patented. In the last 10 years very few design patents have been granted with "dress" in the title of the patent. See http://bit.ly/i6WKe
Fashion designs are, for all practical purposes, NOT protectable under any US body of law. For a good discussion on the topic visit .
Let me break the tie on this issue by agreeing with Daniel and Pamela that a dress design is neither patentable nor able to be copyrighted. Raising it to the level of a trademark is about your only recourse. Certain parts of the fashion industry have long been lobbying to get language added to the law that would allow for the patenting or copyrighting of dress designs but all efforts have failed to date.
It happens that I have been the patent attorney for two clients whose articles of clothing qualified for regular utility patents. One was for a maternity undergarment, an elasticized full body slip: US 6817034, and the other was for clothes suitable for sporting wear, wherein one pant leg is long and the other is short US 5890231.
Separately, I have also helped clients patent a couple of products that are related distantly to clothing, namely a novel hair tie, US 53293884, and a prayer-shawl wedding canopy US5227215.
I don't suggest that typically a clothing design can be patented in the U.S. I have worked with other clothing innovations that weren't patentable. Also, I'd point out that the standard of patentability has been reinterpreted by the U.S. Supreme Court in the past couple of years, raising the bar on the "nonobviousness" test. This was the case of KSR Int'l v. Teleflex.
It is also appropriate to consider whether the clothing design can qualify for a design patent. In the U.S., it would also need to meet a legal standard of "nonobviousness" as compared to the prior art.