I have tried this twice for new accounts and it worked both the times:
1. Go to: http://www.amazon.com/b/r872907?i...2402172011
and sign up to get free 2 credits for new customers only.
2. Confirm that you have two credits by looking at the top right corner of the page. It should say "Credits: 2"
3. Go to "Account Details" and click on "Change My Membership"
4. Scroll down to the bottom of the page and click "Cancel Membership" (don't worry, we are not canceling the membership)
5. When they ask for a reason, select something like "I'm trying to reduce expense" and continue.
6. They will offer you 50% discount for first three months on membership. Take that discount and stay a member.
7. Start at step 3 and this time for Step 6, they will offer you a $20 credit. Take the credit and stay a member.
Following steps are for free third credit. Thanks to brighton and shane0049.
8. Start at step 3 and at step 5, select "I did not like what I listened to" and continue.
9. They will offer you 1 more free credit. Take the credit and stay a member.
Enjoy! Cancel anytime before your first month ends and you won't have to pay $7.50. If you like to keep it, you will get discount on first three months.