Let’s not write letters. Send a note, “Enjoyed meeting you at___.” Do not talk product. If you will see them again soon, mention the event. If not, suggest you have breakfast or lunch in the next 2 weeks to “learn” more about their endowments, trusts, etc. Then be sure these events are not sales-oriented but informational gatherings. Close the meeting with something like, “I have access to some ideas that would help you and the organization achieve its goals. Let’s agree to meet in the next two weeks. I would like to give you an executive briefing I know you will find interesting.” That should do it.