To get the best results from a cover letter, put yourself in the place of a hiring manager and offer information that is interesting and relevant.
Generally, all resume and cover letter mistakes arise from the fact that job seekers don't put themselves in the shoes of the person hiring for a job. The best way to create a good cover letter is to imagine what kind of information you'd like to see if you were the one sifting through a bunch of job applications.
I won't talk about the obvious things to avoid, like typos or grammatical errors. Everyone should know by now that a sloppy resume or cover letter will not reflect well on you. These are more things people automatically do without thinking when composing their cover letters:
Not mentioning the specific job you're applying for
"I would like to apply for the position for which you advertised..." Yes, that actually happens. If someone is filling several positions, he or she is probably not going to take the time to browse through your resume to figure out which position you're referring to in the cover letter.
Using a form letter
If you think you can use the same spiel to introduce yourself to every company you apply to, you're missing out. Your cover letter should mention specific aspects or attributes of the company and why you think you would be a good fit for it.
Repeating the information in your resume
The cover letter is actually the place where you want to let some of your personality show through. The hiring manager has your credentials in your resume. The cover letter is the place to show where your accomplishments will translate to the job being offered.
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