Some resources on writing letters and notes:

Put Your Heart on Paper ... Staying Connected in a Loose-Ends World
by Henriette Anne Klauser

This book describes many ideas for connecting including writing notes, making signs, journaling, resolving conflict through writing and more.

The first chapter includes a letter written from a fourteen-year-old boy to his six-year-old sister on her birthday. He might not have been able to say the same thing to her because of embarrassment or lack of opportunity, but writing allowed him safely say: "I'd like to compliment you on being so bright and cheerful throughout your first five years on this confusing planet. I think you are the smartest little sister a kid like me could ever have. I am so glad you were born."

In the introduction, Ms. Klauser describes her book: "I like to say this is not a 'how-to' book but a 'why bother' book. In every instance, I have tried to show why it was worthwhile for the people in the story to do what they did, and why it would be worth your time to follow their example -- in your own way."

http://www.wendy.com/letterwriting/
An accumulation of letter writing related sites. Topics range from general letter writing to pen pals to collections of letters.

http://co.essortment.com/personalwritel_rvcg.htm
An essay on the five C's of letterwriting.

http://www.angelfire.com/sc/dlecroy1/letters.html
Includes points on "Perfecting the Letter" and "Ten Reasons to Write a Letter".

http://www.weddingusa.com/plan/thankyou.htm
This essay titled, WRITING PERFECT THANK YOU NOTES is specifically about wedding thank you notes.

http://www.drdaveanddee.com/thank.html
Thank You Note Guidelines for Kids. Other Thank You Note questions are also addressed on this site.

http://www.edbydesign.com/wr_strategy.html
Writing strategies to encourage everyone in your family to write -- email or letters to friends, cards and thank you notes etc.