Sample
[DATE, ex. Friday, February 10, 2006]
[NAME, COMPANY AND ADDRESS, ex.
Robert Brown
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate 85245]
Dear [NAME, ex. Robert Brown],
I just wanted to send you a short not thanking you for taking care of my files while I was on vacation.
I know how stressful your own workload is, and I can appreciate that by carrying my additional workload, you probably had very little free time last week.
Again, I'm very grateful for what you did, and I would be happy to reciprocate whenever you feel you need some time off.
Sincerely,
[YOUR NAME]