When you begin your job hunt, one of the first things you should do is make sure your cover letter is up to par. 

Because some recruiters see hundreds of cover letters weekly, it's important to make sure you do whatever you can to make yours stand out. To help, we've compiled a list of cover letter do's and don'ts. 

Cover Letter Dos and Don'ts
Don’t ever apply for a job without a cover letter.
Don’t use a generic salutation, such as, “To Whom It May Concern” when answering a blind job posting.
Don’t waste your first paragraph by writing a boring introduction. Use the first paragraph to grab the employer’s attention. Tell the employer why you’re writing and detail the reasons you are qualified for the position.
When sending hard-copy, don’t forget to boldly and confidently sign the letter, preferably in blue ink. For electronic copies, consider taking a photo of your signature and adding it letters; however, be aware that a signature is not required on virtual cover letters.
Don’t send your letter without checking on how the employer wants to receive it. Often, the answer will be spelled out in a job posting, but you may have to check the career portion of the company’s website or call to determine if the employer has a preference for sending your letter as an attachment, including it in the body of an email or uploading it online.
Don’t miss the opportunity to “brand” your cover letter by matching its appearance to your resume. Use the same “letterhead” as on your resume – with your name and contact information. Use the same fonts in both your resume and cover letter. Make them look like a matched set.
Don’t use clichés such as, “Enclosed, please find my resume.” Employers can see that your resume is enclosed (or “attached” for electronic submissions); they don’t need you to tell them. Such trite phrases just waste precious space.
Don’t send a cover letter that contains any typos, misspellings, incorrect grammar or punctuation, poor syntax or capitalization errors. Especially ensure you spell the recipient’s name and company name correctly. Double-check, as names can be tricky.
Don’t rehash your resume. You can use your cover letter to highlight the aspects of your resume that are relevant to the position, but you’re wasting precious space – and the potential employer’s time – if you simply repeat your resume.
Do send an original letter to each employer rather than a generic letter. Write cover letters that are unique and specific to you and the job you want.
When updating a previous cover letter file into a new letter, do update all the details. Don’t send a letter addressed to wrong person, wrong company or that targets at the wrong job because you forgot to update the details. Chances are the letter will need additional customization to be sure you are specifically addressing the employer’s needs and requirements.
Do send a neat and attractive letter. When mailing a paper copy of your resume and cover letter, make sure you send a clean copy, without smudges, coffee rings or grease from yesterday’s lunch.
Do use simple language and uncomplicated sentence structure. Ruthlessly eliminate all unnecessary words. Follow the journalist’s credo: Write tight!
Do keep your letter brief. It should never be longer than one page, and it’s best to keep it well under a full page. Each paragraph should have no more than three sentences. The letter should be even more concise and to the point when sent in the body of an email.
Don’t be wordy and include irrelevant details.
Do consider enhancing the letter’s reader-friendliness through use of bullets or other special formatting. Even a relatively brief letter can seem like a daunting collection of text-heavy paragraphs in need of a reader-friendly treatment.
Do avoid negativity, for example, pointing out that you lack some qualifications for the job. Be sure your letter doesn’t project a desperate tone of begging or pleading for a job. Negativity never has a place in a cover letter.
Do use a standard business letter format.
Do use action verbs to describe your experience.
Do be sure the potential employer has the information to reach you during business hours. Job seekers don’t always include their physical or mailing addresses on resumes and cover letters these days, but a phone number and email are desirable. 
Do address your letter to a named individual whenever possible.
Collected From Biospace