The cover letter represents your best shot at making a first impression, so make it count. You want to make sure that your letter leaves the best impression of your professional experience while also letting your potential future employer know about you.
Do Not Simply Repeat Your Resume
The cover letter should be a professional look at you as a person, but many job seekers make the common mistake of simply reciting the resume or curriculum vitae already sent. Instead of rehashing and adding details of job descriptions, you could include anecdotes to demonstrate how your skills and experience translate into benefits for the company.
Was there a challenge faced on the job that you can explain in brief that can reveal something about how you approach tasks on the job? Was there a project completed where you excelled? Brief descriptions of such events from your professional life add meat to the bones of your resume.
Tell Your Story, But Not the Whole Story
That being said, do not turn the cover letter into a personal biography. Younger professionals brought up in the social media age sometimes lack a mental filter that would otherwise direct which details are appropriate or inappropriate. Employers do not care about your personal life outside of attributes that will make you a welcome part of the team.
Some details may be better added during the interview process. You should not mention your love of fishing in a cover letter unless you applied for a position related to it. If the interviewer mentions his or her own fishing hobby, then you should briefly mention your common like for the hobby to build rapport.
Ask Not What the Company Can Do For You...
Nothing sounds more arrogant than a cover letter which goes on and on about how the job can benefit the applicant. This leaves the impression of, at best, a subconscious narcissism that rarely makes an employee a productive part of a team. Paraphrase President Kennedy in your own mind and ask not what the company can do for you, but what can your skills, experience, education, and personal attributes do for the company.
Edit and Have a Friend Read It
Most importantly, edit your cover letter diligently. While standards of expression have slipped in recent decades, always assume that the reader is a stickler for grammar and proper expression.
Employers have criticized younger applicants for using slang and even text language in cover letters, so definitely avoid them. Also, if unsure of a word's proper definition or use, avoid it and use a different one. Few things sound less intelligent than the misuse of an obscure word.
Finally, have a friend with writing or professional experience read the letter. Ask them for a critique based on both language use and also the perception left by the writing.