From the fine-print on Apple's website:
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 email@example.com 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.
So, here's what you do to enter the contest as many times you want:
- Go into the iTunes Music Store.
- Navigate to any album, song, or whatever.
- Click on “Tell a Friend” at the top.
- Create a blank message to “firstname.lastname@example.org” and hit the “Send” button.
… Ta-da! 👍🏻