What is the best way to format an xml sitemap?

[Webat]

I am wondering if the urls should be in alphabetical order or if they should be set out in a way that reflects the sites hierarchy?

Thanks.

[RyanKent]

The best resource for XML sitemap information is http://www.sitemaps.org/protocol.php

The order makes no difference at all. If you have proper site navigation, the sitemap really does not perform any function. A search engine will scan the sitemap and determine if it has any URLs which have not been crawled, then decide upon what action to take. The URLs already in their database are disregarded unless they have new information (i.e. the last mod date is newer then when the page was last crawled).