We've posted numerous methods of letting the search engines know that you've changed your xml sitemap.
There are several software and web-based tools that to let Google Webmaster Central and Yahoo SiteExplorer know you've made a modification, create sitemaps as well as custom scripts.
You can even ping them from your browser bar as we noted in an earlier post.
We've found that manually editing your sitemap, where possible, really makes a difference over time. The 'Priority' number should be YOUR priority.
Our tests also conclude that pinging daily is not necessary, just when you make a change to your sitemap.
One of the commercial program providers has a webpage where you can ping Google, Yahoo, Ask and Moreover with your new sitemap.
Take a look at this page from Sitemapwriter (It works!), and use it when you need it. It's free.