Make the letter as creative as you dare. Use short paragraphs for emphasis. Take out every word not integral to the purpose of the communication. Look behind commas to see if the entire phrase is worthy of keeping. Usually it's not.
Adverbs are the scourge of good writing.
Edit out half the adjectives and half the prepositional phrases. Avoid heavy syrup. Make the letter easy to digest. Don't make the letter sound like a rubber stamp. Keep the letter short.