Go into the Extras option on the Main Menu of the game once you boot it up.
Enter in the info they want to sign up for the Shift program.
You'll get a key that way.
After that,
follow Randy Pitchford on Twitter.
Every couple of days or so (or sooner depending on who's keeping tabs on him), Pitchford will post Shift Codes you can enter in the Shift Code menu in said Extras on the Main Menu.
If you enter them in fast enough, you'll get a free key each time.