Some of good advices are the following:
Ensure you include the name of the person, their title, company name, address and any position reference number. This is probably obvious, but ensure that you spell their name correctly, nothing worse than receiving a letter incorrectly addressed or misspelled. It gives a poor first impression.
Before you mail the application letter, read it over again, making sure that it is perfect. Special attention should be placed to ensure the letter:
It is not too long.
There are no grammar or spelling errors.
That you have answered the job requirements.
The application letter flows and is easy to read.