Sample
[DATE, ex. Friday, February 10, 2006]
[NAME, COMPANY AND ADDRESS, ex.
Robert Brown
1234 First Street
Suite 567
Anycity, Anystate 85245]
Dear [NAME, ex. Robert Brown],
It was really nice of you to invite [us / me] to dinner on [DATE, ex. Friday]. I appreciate your hospitality. That is why I regret that [I / we] have previously made plans for that date. {Perhaps we can rearrange for sometime [TIME WINDOW, ex. next week]}.
Thanks again for the invitation. [I / we] look forward to seeing you soon.
Best regards,
[YOUR NAME]