[NAME, COMPANY AND ADDRESS, ex.
Jill Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]
 

Dear [NAME, ex. John Smith], 

I'm not sure how to best say it so here goes:

THANK YOU!

 [CLOSING REMARK TO TIE IN THE ACT, ex. I was able to obtain the apartment as a result of the glowing character reference you gave me].

You are a true and genuine friend and I appreciate very much the thoughtfulness and consideration behind your kind act. 

Best regards,

[YOUR NAME, ex. John Jones]