I have about 10 XML sitemaps with 30000 submitted URL's in each. Indexing is getting down day by day. As of now the URL's are getting submitted on the basis of last modification or refresh date in ascending order to sitemap1, sitemap2, sitemap3......
So if someone has faced similiar issues and tried to research and resolve these issues then please give your valuable comments here.
when you are trying to get your pages indexed you should always make it as easy as possible for spiders. in other words i would suggest avoiding shortcuts and to use the exact formatting that the official sitemaps protocol states.
its not always terrible if you take a formatting shortcut but why not make it as easy as possible and less complicated for search engines?
search engines tend to like fresh content. if your content is static, then search engines will likely crawl you less often so they can save system resources.