This is a tool that I've used again and again on several sites so it's well worth the small investment in purchasing the full version. Here are my tips for using xml-sitemaps.com's sitemap generator.

Installation is easy. Just upload the generator directory and the root files i.e. the sitemap.* files and the ror file.

Go to http://www.yourdomain.com/generator and you should see the configuration screen. If any files need to have their permissions changed then it'll tell you so at the top of the page.

If your website already has a valid HTML sitemap then, on the configuration page, uncheck 'Create html site map for your normal visitors'. This will reduce the load on your server. Your visitors can use the PHPMD sitemap.

Add some login details to the Miscellaneous Definitions (optional section). You don't want any unauthorised person logging in here. It's also advisable to secure the /generator directory so that no one can even see the login screen to this section.

Add any URLs to be excluded in the Exclude URLs box. For example, use the same list as per your 'disallow' listings in your robots.txt

In Crawler Limitations, Finetune (optional section), set the max. pages to 2000 and the max. depth level to 3. This is a good idea because your site may be so large that you'll have to wait hours before you find out if the first crawl has worked or not. You can always increase these limits later.

Save the changes and try the first crawl.

If it only finds the homepage and the server's IP address to the Advanced Settings (optional section) .

If you configured the tool to email you when the map has been created you will receive something like this:

Hello,
you have configured your sitemap generator to receive these automated notifications every time sitemap is created.
Standalone Sitemap Generator Report - 17 July 2009, 10:58
http://www.xml-sitemaps.com
-------------------------
Sitemap details
-------------------------
Request date: 17 July 2009, 10:58
Processing time: 203.54s
Pages indexed: 2000
Sitemap files: 1
Pages size: 33.99Mb

In addition, you can set up a CRON job to re-generate the map automatically every week, month etc.

Now, go to your Google Webmasters account and submit your new sitemap for the googlebot to crawl and watch the visitor numbers increase!

P.S. Xml-sitemaps.com's Sitemap Generator is on the list of recommended third party tools on the Google sitemap-generators page.