All too often, newsletter operators jump straight to referral reward ideas with monetary value. But — most of the time — your audience will be much more motivated to share by emotionally valuable rewards. Rewards that are fun, unique, exclusive and have a great story attached to them.
Insider handshakes are a great example of all the above. People love to be part of an exclusive club (even if it is very silly in a good way).
This is especially true if you ever run or visit events — where a popular twist on the idea is to say "see a member of our team at an event, introduce yourself with our insider handshake, and we'll buy you lunch" or similar.