Good points Cups
but I have some advise...
in my cms I have an image in footer - it calls a local php file, that eventually serves a blind gif.
before that, it puts the ref in a DB table and counts views for that page..
that way, I can even implement googles "priority" field with real user intrest.
And the good point - the site admin does not have to push a button!
if you flush that table reguarly (cron), it will always be quite up to date
each time google collects my XML, the script reads that table and delivers a fresh xml, ordered by the views..
the priority is 1 for top pages and 0.1 for lower traffic pages..
If you want to push the often missed contents, simply reverse the order..
btw - I have a more rudimentary version of this online as a web service somewhere - but I dont know if Im allowed to post the link here..