Make the cover letter specific to that job. This differentiates you from the mass of folks submitting generic cover letters. Most folks want to hire someone interested in this job not just any job. We want people who are interested and excited about the opportunity. Mention something in the job description that peaked your interest. Mention something you've done/learned that aligns to the job description. Generic cover letters are so common that these first two items can at least put you in the upper half (or higher) of the stack.
Show some personality, but don't lose your focus. It can be good to mention how how your volunteer work or travels taught you a professional lesson, but don't spend a paragraph talking about your cat.
Proofread and then have another person proofread. There is always someone who has a typo or grammar error in the sentence about their writing skills.
Double and triple check that you are attaching/uploading the correct cover letter. Name each file with the company name to help you keep track (something like "Jane Doe Cover Letter Acme Corp.pdf"). It seems like there is always at least one person who attaches a cover letter that references a different job.
