How to protect a website Idea?

Have the designer sign a contract beforehand that specifies that any ideas or other information you disclose to them in connection with their potential development of your website belong to you; such information and ideas can't be used by them in any way other than to develop your website, or disclosed to anyone except their employees (who must sign the same agreement) for the sole purposes of developing your website, and that if they breach the agreement, in addition to any other remedies available at law, they are liable to you for all proceeds derived from the unauthorized use and/or disclosure of the ideas or other information.   The above is not intended to be the actual contract, just a quick guide.  If you don't want to hire a lawyer to prepare the agreement, and do it yourself, be careful to try to anticipate any contingency.