Warning- This is illegal. I take no responsibility for your actions and the reactions of the people you try this to.

So, what you are going to do is fairly simple,
Walk into your local store that sells any type of gaming card (Xbox live gold, Microsoft points, psn cards) Any buy any card you would like two off.
Buy that card (yes this is not for free) and take it home,
The next day, take the card back to the store.
Tell the cashier (make sure it is a different one) that the other one did not activate your card.
Show them the recipt to prove the purchase.
here comes the illegal part, take a card off the shelf before hand with out them noticing, and hand them this card when they ask for the one you purchased,
They should not give and fuss and activate it gladly,


remember, i'm not responsible ;).