On a PC, sign into the NowTV site (with your Sky id if you have one, or register an account if you don't).
Go in to "buy a day pass", enter the promo code they email you (and your card details, but it won't take any money - I can attest to that), and you have a day pass (inactive) on your...
I successfully took advantage of this when they were promoting the Manchester derby.
As there were serious problems on that evening, everybody who got the (one) free pass was then given an extra 5 day passes, so I've still got a few left.
NowTV doesn't work on many Android devices, though, and...