I second this very strongly (+1) Sparky!
to the OP:
I'm currently working on my SoP for Lit programs (PhD only, pretty much, unless I do end up sending out MA apps as well) - what Sparky said above is pretty much dead on. No cute childhood stories, keep it straightforward, thoughtful, and focused. Don't use TOO much jargon (just enough that you appear knowledgeable), and make sure that you are NOT SUCKING UP. I.e. - I'm applying to Yale. At no point in my SoP do I discuss Yale's prestige as a reason I'm applying there. I do discuss, in great detail, the professors I'm interested in working with, and how their particular research can help mine (and in what aspect), how the facilities (library and colloquium) can further my scholarship, and how their M.Phil in Medieval Studies on route to the PhD is the main reason I'm applying.
And yes. State that you want to teach and research in an academic setting post PhD. Doesn't matter if it's true or not. Just state it, and believe it as you type it .