When assembling your application, DO NOT, under any circumstances…
- ...include letters of recommendation from your restaurant manager, coach, or volunteer coordinator (unless, of course, you're going for a degree in hospitality, physical education, or social work). We don't need character references; we want to know if you can do the particular work in the discipline for which you've applied. That usually means that all your references should be from professors, and most of these should be in that chosen field or a closely related one.
- ...give as one of your primary reasons for coming to our (middling) program the fact that the school (not even the department!) has a decent ranking in US News & World Report survey.
- ...give as another primary reason the fact that our classes are held at convenient times.
- ...apply to work in a field for which we have no faculty coverage whatsoever. I'm not talking about you wanting to work on colonial Peru and we only have someone working on colonial Bolivia. I'm talking about something like expressing a desire to work on the pre-exilic Hebrews when the department's one ancient historian specializes in the Augustan age of Rome. Perhaps there was a sliver of an excuse for this back 10+ years ago when you had to go to the library and get microfiche of school catalogs to figure out who worked where, but no more.
- ...spend more than a sentence in your statement talking about your hard-luck circumstances, unless it's directly relevant ("My long rehabilitation after the accident prompted questions that led me into the field of disability studies" = OK). If your letter writers want to talk about how you got those Dean's List grades while working to support yourself and your two orphaned siblings, fine. I'll be impressed. But your job is to present yourself as a professional.
- ...turn in a statement of purpose without having a professor go over it. Chances are that you don't understand the genre. Nobody does the first time. Get an insider's opinion so that you don't unintentionally offend anyone, look uninformed, or come off like a pompous jackass. Maybe you're not any of those things, but if your statement makes me never, ever want to meet you, then I'll never, ever know.
UPDATE: So many people have been interested in this that I thought I'd also link to this post from over at Historiann's last December, tackling the same topic. Between the two of us (and our commenters), you should have a nice set of cautionary examples.