For many months, I have been dreading this tip. Including it is like challenging you to find typos in my work. Let me take a deep breath and continue anyway…
When promoting yourself, don’t make mistakes. Don’t misspell words, include grammatical errors, or screw up the formatting of your documents. All of these give a horrible impression to people who do not know you.
It is HARD to be perfect. You can’t do it alone. I find it nearly impossible to proofread more than 500 words of my own writing, which is why I hire proofreaders. For personal documents, I often ask a member of my family to proof my work; then, I proof it myself.
An old boss of mine had a theory, which is that we don’t know why we form opinions about people, but we form them anyway. He meant that most of our inputs are comprised of subtle clues: how the person stands, the tone of their voice, whether they look you in the eye, how they dress, etc. The number and manner of your mistakes is another category of clues.
If you make mistakes in your resume or a cover letter for a job, you are not getting the job.
If you make mistakes in the copy for your web site, you are stacking up the odds against you. Careless mistakes are a big clue in the wrong direction.