Dan Lewis is a lot of things. A former practicing attorney, a company founder, and Elmo’s co-worker, to name but a few. But he also writes a free, daily email called Now I Know, which promises to teach its readers something new every day. In barely two years, he’s attracted more than 80,000 information-hungry folks. What’s his secret?

Early on, Dan asked friends to try out Now I Know and give him feedback. If they didn’t like it, no big deal, they could unsubscribe. But their feedback helped him tweak his work. “The personal aspect of it helped it grow,” Dan remembers. “Friends saw that I was invested and helped me spread the word. In general, endorsements work exceptionally well. I think that’s common sense, but it’s key to appreciate. When people feel comfortable recommending something, and the push to try it is genuine, that makes a huge difference.”

Plugs from websites like Reddit, Metafilter, LaughingSquid, MentalFloss, and Neatorama have helped Now I Know grow a ton, as have tweets from notable folks (like Pee Wee Herman!). But Dan has noticed that huge list growth is often temporary. “Whenever there’s a big gain, it’s followed by a big fall-off,” he admits. “I retain about two-thirds of those big boosts, give or take. Now I Know isn’t for everyone. There’s reading involved—a few hundred words—and it happens every day. That’s a lot for a lot of people. I share stories more than just one-off facts, and many people unsubscribe because they just want the factoid, not the story.”

“Personal” is a word that comes up a lot when Dan talks about Now I Know, which shouldn’t be surprising to anyone who’s read his emails. But that’s just part of his advice for people who want to have an engaged newsletter. “Be personal, consistent, respectful,” he says. “If you really, really like a joke, throw it out, because it’s probably only funny to you. Be honest that you’re figuring it out as you go along, and treat it like a dialogue. Publish with a regular frequency and stick to it.”