Trying to get people to pay for something that was previously free

We're moving one of our free magazines onto a paid subscription model (physical and digital). Like many publishers we have been hit by rising costs vs. decreasing ad revenue.

Many publishers, such as the Guardian, have struggled to raise revenue via subscriptions after being previously free (though The Guardian ask for donations).

We're sending out a letter with our next edition asking people to make a payment for the next 4 issues of the magazine.

Any general suggestions for wording that won't turn people off?

I know personally I have trouble paying for something that was previously free.