You must already have a premium trial to get it free forever
First, You need to download Spotify on your iDevice
Secondly, You need to sign in to your account with the trial.
Thirdly, Navigate to your home screen, press settings>General>Date and Time
Fourthly ,Turn auto off and change the year to 2002. Don’t change anything else.
Fifth Step, Close Spotify
Sixth, Restart Spotify and at the top it should say ‘4020 days left of your trial’
Now,enjoy your free membership
Credit to TooMuchSwagger
Not sure if this is allowed, or if it should be in Diamond section or something