Tips for creating an effective cover letter : Career Development Center, Bahauddin Zakariya University, Multan, Pakistan

Collect your thoughts. Your ideas may not come out logically or sequentially, but write them down. Don't judge and evaluate, simply collect them.
Spend time on your letter. As the adage goes, "With part-time effort, you get part-time results."
Write a draft, let it cool off overnight, and then rewrite if necessary.
Use a strong close, e.g. "After you have had an opportunity to review this letter, I will call you…"
Avoid weak endings such as "I look forward to your reply" or "Please call me at your earliest convenience."
Limit your letter to one page; a letter is an opportunity to sell, so say something about you, while also focusing on the needs of the employer. Write the way you talk. It should be well-worded, concise, and controlled in the use of the pronoun "I".
While a general cover letter can be used, best results come from personalizing each letter to fit the specific circumstances, position, or organization.
Ask for opinions, advice, and feedback from friends, a counselor, or someone in the profession Check spelling and grammar.
Avoid cluttered desktop publishing. Business letters should look conservative. If you want to be creative, do so in your choice of words. If should be aesthetically appealing with careful attention to spacing and format. Use letter-quality printer paper or high quality bond paper. Stick with white, ivory, or off-white.
Remember to sign it personally and include your telephone number and e-mail address.
Don't use someone else's letter and if you are using the same letter for several companies, remember to change the name in the body of your letter.
Devise a system to keep track of the follow-up steps you will take and the responses you receive. Most students have found that binders or file folders for organizing the job search and its correspondence are essential.
Follow-up, follow-up. People will call you, but you'll improve your odds dramatically if you follow up your letters with a phone call.
Don't mark letters "personal and confidential" unless there is a solid reason why a secretary or an administrative assistant can't open them. If your letter is persuasive enough, it will get through.