Ask someone to review your work before you send. This rule applies to every document–from a two-sentence email to a book-length manuscript–that will be seen by someone you hope to impress.
I was once asked to provide a caption for a photograph of a Parisian river scene. Unfortunately, I made a tiny error; a single letter was incorrect. The caption read: “After a busy day of sightseeing, there’s nothing more relaxing than a quiet wank by the Seine.”
It would have taken five seconds to have someone review that little gem. Instead, I got three years of ridicule.