Google Custom Search Engine team today introduces a new way to add new pages to your's search index, in addition to already existing "on-demand indexing", and "keeping a Sitemap up to date"--
starting today, "all verified site owner in Webmaster Tools can simply provide CSE with a page's URL that links to their site's latest content, and Google bots will then periodically visit the URL to discover your site's new content."
"For example, as the owners of this blog, we can provide the URL of the blog's landing page, ditii.com, to our CSE. It will then periodically visit that URL and add any new links to content on this blog to our CSE's index. Since this URL automatically links to all new content on this blog, our CSE's index will always automatically get updated."
You can also provide a URL(s) to your CSE via the URLs linked from a page feature in the Indexing page of your CSE's Control Panel.
"Note that this feature is only for the discovery of new content and does not refresh content that has already been indexed," said the CSE team.
You can learn more about URLs linked from a page, in the help center.