Disclosing in Confidence. To prevent triggering the one-year bar (and to preserve foreign rights), the company must agree your disclosure was made in confidence (and treat it accordingly). It's true that many companies will not sign NDAs, particularly with individuals that they do not know. However, that's usually not the case when the company knows the other party, or has solicited the idea. So, we recommend starting with an NDA (samples are provided here).
Signing your book. Signing an entry in your invention notebook may preserve confidentiality provided it includes a statement to the effect that the disclosure is being made in confidence and the company will take necessary steps to preserve the confidentiality of the disclosure. Still, it's not as ideal as an NDA which may contain provisions for dispute resolution, attorney fees, jurisdiction, or injunctive relief.
Authority to bind. Finally, make sure the person signing the NDA or book has the authority to bind the company (include their title; be wary of "agents") and if in doubt, verify the authority.
If they won't sign anything. Be wary of companies that won't sign anything. Sometimes they have a legitimate reason -- they don't want to be limited in case they're developing something similar independently -- and sometimes they're just using their bargaining power to see what you have without any restrictions. Keep in mind that any "publication" that shows how to make and use the invention, or any offer for sale -- licenses are not typically considered an offer for sale -- will trigger the one-year bar. Disclose sparingly in the absence of a confidentiality agreement.