Both the retiree and his colleagues and well-wishers will be figuring out what to say in a retirement letter.  For those who are saluting their departing colleague, it is important to deliver a speech that fits the character of the retiree, while presenting a proper mixture of sincerity along with some humor.  You might also wish to add a special memento during the retirement letter – such as a poem, famous quotes, or something the retiree may have taught you.