from Apple's fine-print:
1. NO PURCHASE NECESSARY.
3. How To Enter. You will automatically be entered into the Promotion by: 1) downloading a song from iTunes (any free downloads will be deemed an ineligible entry); or 2) a free alternative means of sending an e-mail to Apple at firstname.lastname@example.org via the iTunes “Tell a Friend” feature (a song download or Tell a Friend e-mail will be deemed an “Entry(ies)”). The “Tell a Friend” feature can be easily accessed at iTunes by selecting a song, and clicking the “Tell a Friend” link that is displayed for that song. One Entry will be automatically submitted for each song downloaded or Tell a Friend e-mail sent. The Promotion begins following the downloading of the 480,000,000th iTunes song, and ends with the downloading of the 500 millionth Entry (“End Date”). Only Entries submitted in this time period will be accepted.
Ta-da. So basically, here's what you do to enter the content as many times you want:
1. Go into the iTunes Music Store.
2. Click on any Album or Song or whatever.
3. Click on ‘Tell a Friend’ at the top.
4. Send the message to ‘email@example.com’. (just leave a blank message)
5. And you're all set. :)