Use good quality paper
Mention the job you are interested in up front
Include the same contact information as on your resume
Check and re-check your grammar and spelling - have someone else proof read it
Use a clear font - Arial and Times New Roman are often recommended
Sign your name
Use the same letter for different employers
Use slang or uncommon terms
Send a letter without a resume
Do what you can to find out the name and title of the person that will be receiving the application. You can usually find that out by calling the company and asking who your letter should be addressed to if it's not listed in the posting.
Use Ms. or Mr (Smith or whatever) in your salutation. Refer to the job, where you heard about it and when first. Then demonstrate with your words how your experience and background match the position's requirements. Since you've not done that particular job and possibly not been in that particular industry before, you may want to say that you can see they are looking for someone who has strong communication skills and you have proven excellence. You must give that evidence briefly in the letter. Pick the top qualities they are looking for and give them information that demonstrates you fulfill their requirements.
Let them know that you look forward to meeting with them to learn more about what their needs are. You are excited about the possibility of contributing to their success. Here's where you can be reached. If you haven't heard from them by next week, you will call them to set up a mutually convenient appointment. Thank them for their consideration. Sincerely, your name
This is the opportunity to promote yourself, your writing, your special abilities - and to leave them with the thought that they "must" meet with you. Make sure you use powerful descriptive words, action words. Make sure when they read the letter they can see that you have what they want and more.
If you have specific questions, please send them to me at: firstname.lastname@example.org
The last part will be on Interviewing Skills. Stay tuned.