As a writer, I've learned to tailor my text to my audience. That's why I end up editing a lot of resumes. I find myself giving the same advice over and over (advice other friends gave me when I was just starting out). Here, in a nutshell, is how to sell yourself.
--What makes you the right candidate? Whether you're applying for graduate school, going after a job in a top company, or persuading a publication to assign you an article, know why you're the right choice. Then write it.
--What makes you stand above your competitors? If you're pitching an article about juvenile delinquents, have you volunteered with troubled youth for the past three years? If you're applying at Google, have you worked on open-source projects till midnight? If you're applying to graduate school, have you written an award-winning thesis or done independent research?
--Think about your most spectacular accomplishments, and describe them in specific language. "Worked as a programmer 3 years" won't get you far, but if you add "Improved product speed by 25%" and "Managed a team of seventeen," you're showing what you succeeded at.
--Target your audience, and use only what's pertinent. Nannying experience won't help you land a programming job; your MA in history probably won't help you sell a science-fiction novel.
--You've learned skills from volunteering, from leading amateur groups, from managing your family. Use them if you've got gaps to fill; resumes don't have to be limited to paid work.
Whether you're writing a resume, or an application essay, or a query letter, show off your spectacular self. You can do it--even if you have to stick with the simple truth.