You should not try to keep your startup idea secret. You will look silly doing so *and* end up with worse ideas for lack of input.
Longer version:
Brad Feld http://www.feld.com/archives/2006/02/why-most-vcs-dont-sign-ndas.html
Mark Suster https://bothsidesofthetable.com/on-ndas-and-confidentiality-dfa741ef58cb#.aaq3fk5pn
Chris Dixon http://cdixon.org/2009/08/22/why-you-shouldnt-keep-your-startup-idea-secret/
Hacker News discussing it https://news.ycombinator.com/item?id=779333