XML Sitemaps Are Most Beneficial for Large Sites
XML Sitemaps are not an essential requirement, and are most beneficial for sites with changing or a large turnover of content.
Break XML Sitemaps into Small Chunks
Breaking up XML Sitemaps into smaller groups can give you more feedback on indexing issues, which are reported separately for each Sitemap in Webmaster Tools.
Mobile Sitemaps Are Not for Smartphone Pages
Mobile sitemaps are for feature phone pages only, not smartphone compatible pages. If you have smartphone URLs which are different to the desktop URLs, use a normal XML sitemap with rel=alternate.
Change Frequency and Priority in Sitemaps Are Ignored
Change Frequency and Priority in XML Sitemaps are not used by web search, but the lastmod date is used.
Submit Sitemaps with Changed URLs
If you change many URLs, you should submit a sitemap with the old URLs to Google which will help them pick up the redirects to the new URLs. Webmaster tools will report errors with the sitemap, because it only contains redirecting URLs, but this is fine.