One could allocate each loyal subscriber a limited number of trial passes to share with their friends. The passes would allow them to access one or a few articles for a limited time.
Similarly, LWN.net allows subscribers to share links to individual articles with non-subscribers. (I don't know if LWN.net has a limit, but non-subscriber links to articles are often submitted here on HN.)
I love the LWN model, in addition to this it lets you read the subscription-only articles after a week, and to also (to an extent) choose what you feel you can pay
