Quick Online Tips explains that beyond installing a sitemap on your site, you can help search engines find your sitemap more efficiently:
Ask.com, Google, Microsoft Live Search and Yahoo! have announced support of “autodiscovery” of Sitemaps. The new open-format autodiscovery allows webmasters to specify the location of their Sitemaps within their robots.txt file, eliminating the need to submit sitemaps to each search engine separately.
This step is very useful as webmasters can easily submit their content to the search engines and benefit from reduced unnecessary traffic by the crawlers. The search engines get information with regards to pages to index as well as metadata with clues about which pages are newly updated and which pages are identified as the most important and search users get more fresh content.
Complete details as to how to modify your robots.txt file in order to implement this feature is in the post.