Tips for writing long letters include to engage the reader in the letter's purpose or topic and to write in a way that familiarizes the reader with the letter's topics. It's also important to begin the letter with a positive sentiment. A letter's closing should reflect the level of intimacy of the letter.
When writing a long letter it's important to keep the reader interested and move in a positive direction as the letter progresses. Write the letter in a way that keeps the reader thinking about the topic at hand. If the reader is not familiar with the topic, spend time explaining concepts or defining characters.
If the purpose of the letter is to call the reader to action, use the letter to explain the logic behind the request. Despite needing to explain multi-step logic to the reader, keep writing compact. A long letter should avoid petty complaints, excessive worry or ridicule.
A long letter shouldn't open with an expression that letter writing is a chore. Instead, the letter should begin with an expression that the recipient often is in the writer's thoughts or that it has been too long since the writer and recipient interacted. If the letter is in response to a letter, include an expression of gratitude or excitement about receiving the first letter. If the letter is romantic or extremely personal, the letter doesn't need a long or a formal closing and the letter can simply end with a promise to write again soon or talk later. On the other hand, if the letter is more formal or businesslike, the letter should have a ceremonious closing.