1. Tweak your tone for every company.
Don't send generic cover 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 sass or someone more serious? Figure that out on your own, and tailor your letter to what suits the company.
2. Make a case.
You won't be able to get to this on your résumé, so be sure to make the case for why you're the right person for the job and why you want to work for the company on your cover letter. A good way to sell yourself is to connect your experiences with the job description. List your skills and experiences that match the type of candidate they are looking to hire.
3. 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.
4. 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.
5. 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 instead show how you can be a helpful addition to the company.
6. Stick to the right length.
Limit yourself to a page, and try to stick to four paragraphs. The first should comprise an introduction, the position you'd like to apply for, and a sentence briefly summarizing why you'd like the job and why you're a perfect fit. The next two paragraphs should go on to mention applicable skills and specific achievements that further showcase how qualified you are for the job. You can even use some of the space to explain your suitability if it needs more details, such as why you're changing careers. The last paragraph should be a brief emphasis on why you're excited for the position and a place for you to thank the readers for their time and consideration. You can also make a polite mention of letting them know you'll be in touch.
7. Keep it clean.
Be sure to repeatedly edit your cover letter, and always have at least one other pair of eyes look it over. Check for grammar mistakes, run-on sentences, and spelling errors. Make sure everything is consistent. If you say "I am" in the first sentence, then don't suddenly switch to "I'm" in the next.