Doing a referral giveaway always seemed to me like a fun way to gain new subscribers. My newsletter is for frequent flyers so I decided to give away some air miles.
When a subscriber referred a new person, they earned an entry in the giveaway. The more referrals, the more entries. The giveaway ran for a 7 day period.
Results (actual numbers may differ a bit):
Subscriber count at the start: 2,250
New subscribers: 160 (~7% increase)
Prize value: $350
Cost per new subscriber: $2,19
I used Sparkloop which costs $30 / month for my amount of subscribers. It's easy to set up and their support really helpful!
What I think I learned
Someone made a comment about how he already referred the newsletter to friends before the giveaway. And that it would have been nice if I did it sooner. This made me realize that I should watch out for doing this too often and reward the current subscribers for reading my emails.
I think there might be possibilities for a sponsored giveaway when the newsletter gets bigger.