The best way that I have found to get credits is to complete the offers and watch videos
It takes some digging and a little bit of time but you can find offers that offer that all you need to do is sign up with a Credit Card but it doesn't actually charge you. Then all you have to do is cancel he subscription and close the account.
The only offers that I have actually "Paid for" are the flowers offers, if you have a significant other, use the FTD offer, its under the get free credits tab.
Wife gets flowers "just cause i love her" and in return...I get credits and bonus points with her to use at a later date. Win Win IMHO
or you can just buy credits, there's always that