This is a tip from Craig Mod (in a post full of great tips – check it out!): it doesn't take much effort to reply to new subscribers and sometimes recipients are disproportionately impressed that they heard from the publisher.
It's literally as simple as setting up the "You Have a New Subscriber!" email notifications that most newsletter platforms offer and then, um, replying to them.
An extra twist on this is when someone buys a gift subscription – you can write a quick thank-you note to the person who gave/sent the gift as well.