I can't believe I have to say this, but proofread your cover letters! Over the weekend I had two editors complain to me about the number of cover letters they've received with spelling and grammatical mistakes, and it's not the first time I've heard it. I have to say, I'm flabbergasted. For whatever job you're applying to, but especially for jobs like editing, you have to hand in an application free of mistakes. A big part of an editing job is ensuring an error-free publication; your cover letter and résumé are the first indication of whether you're capable of doing that. Having mistakes shows not only that you can't do that, but also that you don't pay attention to detail, you're sloppy and that you don't have an understanding of the job you are applying to. I thought proofreading was common sense. But, since apparently it's not, I'll say it again: proofread your work!