Writing Letters - Formal Letters

When writing a formal letter, it is important to follow the right format and structure. You start with your address at the top right of the page and the address of the person you are writing to on the left of the page and below your address. The date should be written on the left of the page above the address of the person you are writing to. 
Below the addressee, you skip a line and then start with the salutation. If you don't know the name of the person you are writing to, use "Dear Sir or Madam," whereas if you know the person's name you write "Dear Mr Smith" or "Dear Ms Smith". Use the title "Ms" if you don't know if the woman is married or unmarried. 
The first paragraph should start after skipping a line under the salutation, with no indentation. Here, you state your reasons for writing the letter. This should not be more than a couple of lines. - "I am writing this letter with reference to ..."
In the next paragraph you give more details about why you are writing. Give relevant information, but don't expand too much. Use linking structures to make your letter flow. "Moreover", "Furthermore", "In addition" are all good examples of words which can be used to link sentences together.
The last paragraph consists of telling the person what you would like them to do as a result of the letter, whilst also thanking them for their time.- "Thank you for your time. I hope to hear from you at your earliest convenience"
When signing off, look at how you started as a guide. If you don't know the name of a person you can sign off with "Yours faithfully," and if you do know the name of the person you should use "Yours sincerely,". Make sure to leave some space after this for your signature. Your full name a surname should then be written under this. 
It's very important to write simply, clearly and logically in formal letters. Contractions should not be used ("I am" not "I'm"). In a business setting you must keep in mind that the person reading it will have only a limited amount of time to do so. Therefore, keep your letter short, direct and to the point. 
Some useful abbreviations:
ASAP : As soon as possiblecc : Carbon copy- used when you are letting the person know that you are sending the letter to more than one person. enc. : Enclosure- used if you are including any additional papers with the letter. 
Citation:http://www.englishforums.com/content/lessons/writing-letters-formal-letters.htm