Look, there's nothing wrong with the typical approach. But it's also forgettable because it's clear you sent the same message to everyone.
Where's the personalization? Where's the "wrinkle?"
Example: "Thanks again for coming to the [name of event; for instance, "Hill Valley grand opening] this past week. I'm glad you were able to attend and [the reason you're glad; for instance, see our new store firsthand"].
[Then, the special reference; for instance, "Plus, wasn't the catered food delicious? It was so funny how we 'fought' over the mini grilled cheese sandwich. Next time, it's mine!"]
[If you don't have a moment the two of you shared, be specific about another topic; for instance, "I know you're not the biggest fan of country, but I hoped you still enjoyed the twangy country singer who performed!"]
The person expected a quick, generic thank you. Instead, you reference a specific moment from the night that you both shared. It takes the message from "that's nice" to "wow, what a special note."
In every writing scenario, the goal is to be unforgettable. The "wrinkle" method will make it happen.
Danny Rubin is the author of "Wait, How Do I Write This Email?"
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