It has been said that the letter is a dying form of communication, a concept replaced in our hearts and minds by e-mails, phone calls, text messages and a variety of other modern communication methods.  That may be the case in general, but it is still true that for some purposes - such as lodging a formal application or a complaint - the letter remains the most direct, acceptable and useful form of communication.  Indeed, there remain cases in which the letter remains the only acceptable form of communication.

With that said, it is worth thinking a bit about what makes up a good letter.  The key to this is claritiy of purpose.  That is, if you want to write an effective letter - whether a cover letter for a job application, a formal complaint to a company or a correspondence to your local congressman or congresswoman - you need to hold a steely focus on the job at hand.  You should begin the letter simply and directly by stating exactly what it is you are writing about.  By all means elaborate about this in the following few paragraphs, but get rid of anything not crucial to the purpose of the letter.  Extra sentences will (at best) merely waste your readers time, or (at worst) actively confuse your message.

At heart, the thing to remember is that only clear, accurate communication is listened to - and that, of course, is the goal of any letter writing.