Useless tips for new JETs
I say "useless" because there's not much this vague advice, or any other vague advice of this kind, can do for you. Practical tips are much more helpful. You'll read this and instantly forget it, and then fall victim to all the things I tried to warn you about, because that's the only way to learn anything. You've got a good heart, though. You'll be alright.
You can’t learn Japanese by immersion alone. I had a vague notion that I would pick up tons of Japanese just by living here, and to some extent I did, but you will hit a plateau without focused study. And studying the language in an immersive environment is so much more fun and efficient than studying it elsewhere! There were a lot of times I'd learn a new vocabulary word or grammar structure, and then catch it being used in the office the next day.
Travel as much as you can (if you want). I say if you want because obviously there's nothing wrong with staying home and exploring your local community. But if you are interested in traveling, you'll have to make plans early and take advantage of the holidays/breaks you get. Don't put things off towards some vague future. Aside from your time on JET being limited, Japanese people tend to book things quite far in advance, meaning that if you wait, prices may rise or things may be completely booked out. I definitely recommend doing as much domestic travel as you can, and a little international travel as well. I think it was the thing that most helped me fight off the lows of culture shock, by constantly reminding me how beautiful Japan is and how many things there are that I love about it.
Use names often. I know it sounds super unnatural to be saying someone's name all the time when you're talking directly to them, but your students probably won't really recognize that if you're speaking in English. They'll be happy to realize you remember their name, and the practice will help you remember as well.
Don't let the system get you down. Obviously there are flaws with the ALT program and English education in Japan as a whole, but unless your plan is to become a policy maker, talking in circles about it and letting it dominate your thoughts probably isn't the best use of your time and energy. Despite its limitations, this program is still an awesome opportunity to see how a Japanese school works, get to know and teach and hopefully broaden the horizons of some really incredible students/people, and of course experience living in Japan. It's definitely an unusual job which took a lot of getting used to, but in the end I think it was a good one.
This job is social af. Not that I want to reinforce exaggerated introvert/extrovert stereotypes on tumblr dot com, but fellow introverts, take note. Teaching is already an extremely social job, and on top of that JETs are expected to be friendly and outgoing with everyone they meet as part of their "cultural ambassador" role. I'm not saying this to scare anyone as I do think it was really fun and has helped me grow bounds as a person, but remember to take time to recharge on your off hours. Don't feel bad or wonder if you're not doing a good job if you can't seem to keep up the same social obligations as others.
Which seems to lead me into the next point: don’t compare yourself to others. Everyone has different strengths and weaknesses, and sometimes JETs seem to get weirdly competitive with each other rather than supportive. There really isn't one model of what an ALT should be, so if you find yourself thinking that you aren't as strong a teacher, or not as good at making local friends, or not as funny and entertaining, not as involved in the JET community, not as far in learning Japanese--step back and think about what positive qualities (which you most definitely have) you can contribute, and which you probably already are without realizing it.
On that note, don’t compare your situation to others' either. Yes, it does suck that there seems to be so little structure in place to control what kind of resources/support/job JETs have when we all sign up for the same thing without knowing what we'll get, but on the other hand it's just not realistic to expect thousands of people scattered across the country to be living the same experience. It's hard to resist getting jealous of someone else's friendlier staff, more fulfilling work, more affectionate students, cheaper rent, more convenient or exciting or beautiful placement--but that stuff always has been, always will be there in life, and is just dragged into the spotlight on JET because you have no control over what placement you get. It's actually a really good opportunity to reflect on your relationships with envy, control, and desire, and consider what changes you want to make to them going forward in life. Get thee to a temple and meditate it out.
It’s lighter than you think. My final note here is stolen directly from Sister Corita Kent's rules for students and teachers, and is presented without comment because there's nothing else to say: be happy whenever you can manage it. Enjoy yourself. It's lighter than you think.