<p>They should use very specific examples of what you have done or what your “approach” is, not just general statements about how you are good. They should discuss what makes you unique. For that reason, it is usually best to choose a teacher who knows you the best, not necessarily someone in whose class you did very well. </p>
<p>Also, if someone in the wider “community” (i.e., not a teacher or staff person at your school) knows you very well or has worked with you extensively, for example on some kind of research project, performance or major endeavor of that sort, consider sending a third letter if you think they would help illuminate you as a person.</p>
<p>Oh, and did I say SPECIFIC examples? Think about this, because the adcom is reading thousands of these letters and they can sometimes all start to sound the same after awhile, unless they are very specific - time, date, place, who, what, how, why. An action (or example of one) speaks louder than 1,000 words.</p>