When to consider a no index.

There comes a time in your websites young life that you need to make some adjustments to the code that resides on the pages. Whether it's adding images, deleting pages, or you need to adjust the frequency the robots visit your site, or what pages they follow or index.
The no index tag can and should be applied to log-in pages or credit card info page. This way when the search engines index or try to index the page there is no way that the search engines will look for an entry point past that page. However without the no-follow tag it will still follow all text links and keeping the links associated with the content on the site. Use no-follow tags on: Log-In pages, contact us pages, internal search pages or maybe even examples of what not to do on a site, show the user what not to do, and tell the search engine you do not approve of this site in lesser words. Use the no-index for any of the pages of your site you do not want the search engines to find. Whether if it's duplicate content that you are just trying to provide the "users" the proper information, or if its a page that changes it's content frequently and the previous information is not archived. Specials pages can be a page that does not indexing due to the frequent change of content on the page, rankings will never be achieved when the content is always different.
Do not put a no-follow tag on your links page, you want the search engines to find the other sites that you are linking to, this will establish a chain of data streaming to the people that use your site.