Today I build one PHP script to create sitemap files for websites on my office. This is not so great script, but help me much to finish the tasks 
With this script, I do not need to enter the URLs of the website manually, that is contain thousand of URLs. The script will scanning for all files on the website, and then write it on XML file, that suitable with Sitemaps.org protocol.
If number of URLs reach the limit (50.000 URLs), it will automatically split to another sitemap file.
If you want to try the script, just download it here. You need to configure it first, to be suitable with your server.
After the files are created, you may try to validate it. I use this tool: http://www.on-the-matrix.com/shared/XmlValidator.aspx
Remember, you have to specify the schema used for the validator.
Or, the easier way is validating by this software: XMLBlueprint
The tool is not free, it is 10 days trial. But you need it for hours only
If validated, you may submit the sitemap index file to search engine, like Yahoo:
https://siteexplorer.search.yahoo.com/submit
Happy submitting
