This is directly from the Apple site.
"By default, the iTunes directory reads every feed once per day. The subscriber's iTunes client picks up new episodes based on the preferences set by the user.
If you need to request that iTunes update information about your podcast immediately, you can ping the iTunes server in one of two ways: using an XML POST or by entering a URL in a browser.
The XML POST should be addressed to
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/pingPodcast and formatted as follows:
<?xml version="1.0"?>
<methodCall>
<methodName>weblogUpdates.ping</methodName>
<params>
<param>
<value>PODCAST_NAME</value>
</param>
<param>
<value>http://FEEDURL/</value>
</param>
</params>
</methodCall>
where PODCAST_NAME is the title of the podcast, exactly as it appears in the <title> tag, and FEEDURL is your podcast feed URL.
You can also ping the iTunes server to update your podcast information by entering the following URL into a browser:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/pingPodcast?id=FEEDID
where FEEDID is your iTunes podcast numeric ID. You can also use:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/pingPodcast?feedURL=http://FEEDURL
where FEEDURL is the url for your feed.
Regardless of the ping method you use, iTunes will return an HTTP 200 code.
"
There are other tags that define your artwork file etc.
HTH
Brian