1. Tweak your tone for every company.
Don't send genericcover letters that can be used for any job application. You want to make sure that the tone of your letter fits the type of firm you're applying to. Is the company looking for someone with sassor someone more serious? Figure that out on your own, and tailor your letter to what suits the company.
2. Be different.
Don't repeat everything on your résumé in your cover letter. The letter is your chance to shine and show a bit of your personality. Repeating what's already been said just takes up valuable space.
3. Don't bring up your weaknesses.
If no one is asking you what your biggest failings are, then don't be so eager to volunteer that information. The cover letter is not the time to reflect on self-improvement; save that for the weakness question you may be asked during the interview.
4. Focus on the company, not yourself.
Try your best to show that you care about the company and how you want to help it grow. Try to avoid using too much "I," and show how you can be a helpful addition to the company.
5. Stick to the right length.
Limit yourself to a page, and try to stick to four paragraphs. The first should be an introduction, the position you'd like to apply for, and a sentence summarizing briefly why you'd like the job and why you're a perfect fit. The next two should go on to mention applicable skills and specific achievements that further showcase how qualified you are for the job. The last paragraph should be a final brief emphasis on how you're excited for the position and a place for you to thank the readers for their time and consideration.