If you haven't already, check out the US patent office web site & search for your idea to see if it's been done already ... go to http://www.uspto.gov/patft/index.html
I like to use the quick search under the issued patents field. I agree completely with Will Call, a non-disclosure agreement us usually more than sufficient.
A full blown patent is expensive so some of the alternatives are:
*Provisional Patent - lasts one year to prove out design.
*Patent application - some people swear by it ... just send in your application, if it's filed in the patent office you have an official date!?
*Keep it to yourself & do the design work & prototypes yourself - there are local inventing groups in many states set up by county or city that may be able to help. Here's one for example http://www.juneaucounty.com/economic.asp?cid=251&id=252
*Most prototype companies are not waiting for you to come in so they can steal your idea, they make money building prototypes. Not as risky as many seem to think.
One thing to keep in mind is that if you don't do it someone else eventually will. You sometimes have to take risks ... big risks - big rewards. It may be hard to swallow but even a stolen idea feels better than one not done at all ... I speak from experience.