Mind the format. The date should appear two inches from the top of the paper on the left. Drop down two lines so that you leave one blank line, and add the recipient's name, title and business address, all left justified. Leave another empty line between the recipient's address and the salutation. If you are not using letterhead, position your business address above the date, left justified. Add other contact information, such as your email address and phone number, one line below the signature line at the bottom. The left margin should be 1/2 inch wide. Allow 1 inch for the right margin. Proper formatting signals respect and seriousness.
Use the addressee's honorific or title ahead of her name. Some cultures are particularly sensitive about this. In the United States, in some situations it is appropriate to write "Dear Paul Johnsdown" in the absence of any title. But in India, for example, you must say, "Dear Mr. Johnsdown." Always refer to Indian business colleagues as Miss, Mrs. or Mr. if the recipient has no other honorific. Make it your business to know the customs of foreign recipients.
Get to the point without a long introduction or chattiness of any kind. Keep it brief and clear. Start with one friendly sentence and then get to the main point. After the first short paragraph,explain your rationale for the main idea, and in a succeeding paragraph, provide the reader with necessary details and supporting data. Most business letters should be no longer than one page. Rambling will annoy the reader, compromise your reputation as a focused business person and signal that you don't respect the reader's time.
Leave white space between paragraphs. This technique makes the text much easier to read and is a courtesy to the reader.
Proofread. Sending a first draft to any colleague or business associate says that you don't respect the reader, the reader's position or the subject. Send only final, polished drafts. If you need to have someone else look at the letter for you, do so. Also, double-check the spellings of names. "Smith" for example, is also spelled "Smyth."
Type out your name in the signature line, but also sign the letter by hand, which is good manners.