"The deepest craving of human nature is the need to be appreciated," said philosopher William James. Someone who helped to organise a retirement party for you, or who entirely organised it, probably went to a good deal of trouble to do so and deserves more than a casual word of thanks. A well-written, formal thank-you note will acknowledge the effort and give the organiser a memento of your good feelings.
Make a short list of moments at the party that particularly amused you or made you feel good. It might include the way you felt when you walked in the door, when you saw that many people care about you or a special moment you shared with one guest.
Open the note with a straightforward comment thanking the note's recipient for the party. The entire first paragraph need only be that one sentence. "Thank you for the wonderful party" will do.
Draw from your list to write the next paragraph. Describe how those special moments made you feel and list them specifically. For example, "I was so happy to see Jane Jones there, even though she left the company 10 years ago." List one or two other moments. Mentioning specific moments sincerely suggests that the party made you feel good and left you with happy memories, which is exactly what someone who plans an event like this wants.
Acknowledge the effort and thoughtfulness that went into planning a party that pleased you so much. Be sure to mention the note's recipient by name, as recommended by in an article in The Morning News on thank-note writing. "I know you went to a great deal of trouble to plan this party, Dan, and I want you to know that I noticed. It couldn't have been a more meaningful evening."
Conclude the note with one sentence that reiterates your gratitude, and sign it.
The Rutgers Office of Financial Aid advises thank-you note writers to use quality paper and says that it is permissible to handwrite or type it.