If you wish to thank someone for participating in an event, sending a carefully thought out thank you letter will help you impress the person who has helped you while emphasizing how much you value his contributions. Not only will your acquaintance feel appreciated, he will be more likely to help you again in the future.
Specifically mention the action for which you are saying thank you. Generic thank you letters may give the impression that you have not paid sufficient attention to an individual's contributions to record them correctly. Impress a card recipient by recounting specific details of how helpful she has been.
Use polite language, but avoid sounding formal for the sake of losing warmth in your tone. For example: "Dear Jane, thank you so much for your help on Wednesday." Be sincere and convey gratitude without exaggerating or accidentally patronizing the recipient.
Be succinct. Thank you cards should not exceed two or three sentences. In your first sentence, express gratitude and summarize the recipient's contributions; in the second, convey how helpful this assistance was to you. If you know the recipient well, you can add a third line to say something humorous, add a more personal sentiment or reference an inside joke.
Send the card promptly after the recipient has participated in the activity that requires thanks. The person will feel appreciated if you send your thank you card within 10 days of receiving his assistance. If you wait too long, he may feel taken for granted, or that you have forgotten what he did for you.