Learn English

Japan is a stickler for the technical and grammatical parts of English, so to teach English in Japan, you need to know those parts. Honestly, as a "grammar nazi," I think you ought to learn this period. People do judge you for poor grammar. If you really want to JET or find another way to teach English in Japan, you need to put the time in to know what you are talking about. I think teaching is incredibly important, but you can only teach well if you know what you are talking about.

Learn your commas! Comma splices are disgustingly common and English and a lot of us don't notice.

Learn the correct words!!!! Their/They're/There (check the Oatmeal out, he'll help you :P) To/Too/Two Your/You're

Grammar! Please? Read what you write: if you sound like a dumbass (mix up is and are?), then readers will think you're a dumbass.

Look up what an object is! It matters

Just generally take the time to really learn the language you want to be teaching. No matter how cool going to Japan is, you are going to teach and that is a sacred goal. Also, if you have a grammatically correct SoP, you'll look better because your non-standard English won't damage the readers' opinions of you.

I know that JET never says you have to be an English teacher in general, we are assistants (or will be) and English isn't something that you must master to be in the program, but you really should anyways. Knowing your grammar, knowing your language, knowing how to write are all so important in everyday life. You can argue that you'll never need algebra (I agree), maybe even ignore history, but knowing how to write and present yourself is endlessly important. So, present yourself well and give your students the chance to do the same. You don't want to misuse their for there and mess up your students' education, do you?