1) Doing it yourself. You may be a good writer, but you have to be extremely knowledgeable in the use of key words that employers will scan for. Key words will vary based upon occupation, industry and position level.
2) Having a friend or relative prepare your resume. They may have good intentions, but they can do more damage than help.
3) Preparing a resume that reads like a job description vs a marketing piece.
4) Failing to demonstrate the value you can bring to an employer.
5) Using an online resume template.