State the purpose of your formal letter in the first paragraph and don't veer from the subject. Try to avoid flowery language or long words. Keep the letter short and to the point. This excerpt from Strunk and White's The Elements of Style (4thedition) provides the perfect rule of thumb:
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all sentences short, or avoid all detail and treat subjects only in outline, but that every word tell.