One easy solution is to have a good NDA, and have it signed by every person you tell your idea.
Practically, however, it has limited utility. Many VCs and technology companies do not want to sign an NDA (because they might be already working on similar ideas).
The second solution is to reduce your idea to the most marketable practical embodiment (does not have to be a real product, just a detailed description), and patent it with a good patent attorney. I don't recommend doing a patent yourself or through one of those quick on-line services. If you are serious about your invention/idea, you can get a good patent attorney do it well, and can reduce legal costs by providing detailed description and drawings to the attorney.
A simple patent can be done at fixed cost of about $3.5-6K, depending on the length and technology.
-Daniel Basov
Solton Rosen & Balakhovsky LLP
110 Wall Street, 11th floor
New York, NY 10005
212-344-1744
www.srblaw.net
