Nokia announced a new deal for everybody who wants to lay their hands on one of the select Lumia devices. If you buy a Nokia Lumia 521, Lumia 925, Lumia 928, Lumia 1020, Lumia 1520, or the Lumia 2520 by January 31, you are eligible to receive a $20 voucher, which can be redeemed at the Windows Phone Store or the Windows Store.
Once you've purchased and activated your new device, you have to create an account on Nokia's website, register your device, and then you will receive an email with a unique code for a $20 Gift Card. The promotion is for US citizens only and they can benefit from only one voucher per user account.
Note that you will have to register your activated device by February 15 in order to be eligible for the promotion. Also take into consideration that it might take up to 3 days for your activation to be properly processed. Once you've done so, you will have 180 days to spend the $20 Gift Card on whatever is your cup of tea. More details about the terms of the promotion can be found here