That said, you need to have a cover letter. Just sending a resume says you cared enough to send the very least. And there are dozens of others who did more than that, so out you go with the first review.
Your cover letter should state your interest in the position and why you believe you will add value. This should be followed by an overview of your skills and experience that supports your statement, preferably in bullet points or some other extremely simple and quick to read format.
Then a closing paragraph that reiterates your interest and explains any logistics (am now living in Philadelphia but will be moving the New York where the job is in 3 weeks) and that you would be thrilled to move forward in the process.
Lots of white space.
Lots of bolding and underlining to focus on the key points.
Everything but the first and last paragraphs can be templated.
Your can spend 6 to 8 hours crafting every letter perfectly every time you apply for something. But I guarantee you no one is going to look at it for more than 6 to 8 seconds before they make their mind about you.
So why bother? Be cogent and templated for 80% of your cover letter, customized for 20%. This will allow you to get as many as possible out there, and that will increase your odds of getting interviewed .