Be sure to add a drop-shadow. If these are done well, the recipient may even reach out to detach the note. Some marketers have found success by turning their recipients into co-conspirators. A wild promise, with no payoff, results in bad faith. Do offer the opportunity to win something.
Contests are perennial favorites.
We all love getting something for nothing. Remember, your envelope is a silent salesperson. You can help get that envelope opened with the right choice of words. Comment on this article here . You can learn more about Guerrilla tactics from the Guerrilla Marketing books and software and our Daily Guerrilla Communiques on marketing, selling and online marketing.