I'm about to write a statement of purpose for graduate admission and my advisor passed along to me the following pieces of advice to keep in mind while writing (based on his style and experience in admission commitees).
I completely agree with every point he has made. However, I'd like to have further insight and suggestions and yet another reality check on the soundness of these pieces of advice.
Advice from my advisor:
Don't try to sell me my own research area by explaining to me how fascinating it is. Do show your interest by showcasing your previous experience working on (or studying) related topics (not necessarily very closely related).
Don't tell me how amazing, prestigious, perfect my institution or research group is. I know more than you do about that. Also, chances are that you are applying to a lot of schools and don't have inflexible interests yet, which is fine. Just tell me that you may be willing to work with a certain research group that seems more or less in line with your previous experience or future directions.
Don't tell me a cute story about how you fell in love with my field when you were a kid (or high school senior or college freshmen). I just don't care. I want to hire a soon-to-be professional to join my research group, not a little boy with a cute story. I only need to know factual information that show your commitment to the field (for example, courses/conferences attended, projects undertaken).
Ban ill-crafted, pseudo-literary, flowery, cheesy narrations. Just write facts. Straightforwardly. Succintly. Accurately. I'll draw conclusions for myself.
Ban any buzzwords, meaningless adjectives and adverbs. If they don't add concrete information, but are there just an ill-advised attempt to impress, cut them!