Check spelling and grammar
The moment that your reader spots an error, your credibility will start to slip. The more errors a letter contains, the more distracted your reader will be from your message. Double check everything--your facts, spelling, punctuation, grammar and mechanics. Don't give your reader a reason to conclude that you're careless and your letter doesn't deserve much attention.
Stay on track
Be brief and stay on track. Don't wander from your main subject. Make sure that you understand your topic and can discuss it with authority. Use forceful, positive statements about your direct experience and knowledge.
Stamp out and eliminate redundancy. If you reread your work, you will find on rereading that a great deal of repetition can be avoided by rereading and editing, so reread your work and improve it by editing out the repetition you noticed during the rereading. (You should notice a great deal of redundancy in the preceding sentence.)
Reread your letter
Many people write an important letter, print it out and mail it without further consideration. If possible, let your finished letter sit on the computer for an hour or two, then go back and reread it. Do not skim your letter. Read it closely and you may be surprised at the number of simple mistakes you will find. If you do find mistakes, correct them and read it again. Can you read it through without finding more mistakes or rewriting any sentences? Have you stated your ideas as clearly and concisely as possible? If not, then your letter is not ready to mail. It takes patience to write a good letter. With time and experience, however, your letters will begin to flow more quickly and easily.
Do not overuse quotations. You should use other people's words and ideas to help craft your letter, but you should never string together quotations to fill paragraphs. You also should beware of trying to tie together or abbreviate quotations with excessive ellipses. Use these sparingly. You should only quote those things that are particularly well-worded, or peculiarly worded, or are worded in such a way that you want to emphasize them in your argument.