I`m kind of surprised that nobody`s replied yet. The textbook answer for this is to file a patent application. If your idea is novel and non-obvious and the application is well written (from a legal standpoint), you will be able to prevent them from taking your idea.
There`s a lot to know about patents. The filing fees for a provisional application for a small entity (like yourself) are quite small (~$100), though the application expires in one year, requiring you either to give up the idea or to pay for a non-provisional (and possibly foreign) applications at that time. This may be your best bet and then you`d have a year to try to have a company license or buy your idea and pay for the other patent fees. Writing a patent can be tricky. You need to fully describe your invention and there are some terms and phrases that should definately be avoided. Depending on your budget, consider hiring an attorney. Alternatively, read up on writing patent applications. Importantly - be sure that your idea is new and nonobvious. Otherwise, filing the patent application is unlikely to help.
Other possible solutions include having the companies sign a non-disclosure form, though most companies are extremely reluctant to do this.