A few days ago I wrote that your resumé should make no sense, where I explained that you should customize your resumé so that it is tailored to the receiver to such a degree that sending it to anyone else would make no sense.
A cover letter is similarly customized, but I think that’s more obvious. Never use a generic cover letter. If the application is cold, you need all the help you can get. If it’s not, for example, if you are being referred, then you really need to mention that.
The cover letter is a response to the job description. It is a point-by-point argument for why you are best person for the job as it was described. It is a guide for navigating the resumé. It is a writing sample. Every job description asks us to have excellent communication skills. Your cover letter is the way you prove this.
Before you even start to write it, you need to dissect the job description and pull out the most important asks where you have an excellent response. If you don’t have an excellent response, this might not be the job to apply for.
You will also have one or two things about you that are so great, that you need to get them in the letter. This could be a side-project or open-source contribution, a talk you gave at a conference, some technical achievement at work.
Finally, you should research the company enough to try to find other things that would make you a good fit. Read the company blog or twitter. Get a sense of who they are. If the company is small, you might be able to find team members or the hiring manager of the job they are looking to fill.
With all of this raw material, write a letter that makes it clear that
- You are specifically trying to get this job, not any job
- You are willing to put in work to get it
- You have the required skills of the job
- You have excellent communication skills
From experience I can tell you that getting one like this is rare. When I am the one filtering the pile, I will always call anyone that does this and give them a chance.
I got my last two jobs with cold applications and extremely custom letters. Of course, the letter just gets you the interview, but you are going to do a similar amount of preparation for those as well.