The following advice has been collected from both current and former JET's in Oita prefecture. Please keep in mind some of this advice may not be relevant to your situation, and it is up to your discretion whether or not you follow it.
Be ready to have to rely on others. Even if you're fluent, there's bound to be something you need help with. Don't be afraid to ask.
Don't be a d###. You're here to learn, not preach. Have a good attitude.
Don't compare your situation to other JETs. Every placement has its benefits and disadvantages.
Don't take yourself seriously and have fun.
There will be days where you love this job and there will be days where you hate it. Try to focus on the good points of your placement. You'll hear a lot about your friends' placements and you'll think, man, I wish I was you. I can guarantee you that they have their own problems to deal with. That doesn't mean that you can't vent about your problems to your friends or the older JETs. A lot of them probably are dealing with the same thing or have in the past and have good advice
Always ask. If you don't know what the flyer on your desk means, ask someone. If you think you heard someone talking about sports day coming up, ask them when it is, what the schedule is, etc. Never expect for someone to come and tell you everything unless you are okay with being notified just minutes before whatever it is happens.
Don't completely bubble yourself in the JET community. Make Japanese friends too! Go out with your teachers, chat in Japanese to randomers in bars, meet a Japanese guy or girl. You'll meet some wonderful people and your JET experience will be missing something special if you don't.
Start studying for the driving test ASAP (if applicable).
Travel, meet with other JET's, socialize, don't be afraid to go out by yourself.
Don't stress about getting your apartment filled immediately.
Get a laptop, get Amazon Prime, buy some stickers, chillax.
Get a bike as soon as you can and explore.
Create healthy habits: exercise, read, write, and learn Japanese.