I was quite shocked how many emails and cover letters from candidates were 100% generic, had spelling mistakes, highlighted irrelevant skills, didn't respond to basic questions on the job ad etc.
My top tips would be (in this order):
* READ the job ad carefully.
* Write an email / cover letter that 1) shows that you've read it. 2) is specific to both the company and the role you're applying for. and 3) answers all requirements on the job ad.
* Try to focus on what makes you a good candidate for the role and company. The focus should not be on your skills however, it should be on the required skills that the company needs, and how you could fit.
* Read the job ad again :)
* If possible, adapt the resume as well to highlight those areas of good-fit. Or at least highlight them on the cover letter/email, so the person reviewing your application would even be interested to read your resume...