One thing which will probably occur to you pretty quickly in this job is that not a lot is usually expected of you.
You can potentially change that, though I wouldn't guarantee it. This entry is about how you can use your free time at school.
If you put in the effort and show some well thought out games and activities, or show that your teaching ability/classroom control is high, you may be given more responsibility.
However, sometimes, however much you try, you still have several hours a day with literally nothing to do.
Some people love this, some hate it. You can use it to go on Facebook if you want. You can learn Japanese, write a book or even study for a university course.
You can also, of course, use at least part of it to evaluate your own teaching skills. We all have weaknesses in some areas, and taking a step back (a particularly useful tool here is the video-taped demo lesson) to assess where you can improve will not only help you sharpen up your game, it will also make you feel better.
That last one is an important point. It's very common for the ALT to be left out of school (and even lesson) stuff to the extent that we sometimes wonder if we're needed at all. If you choose to accept that and follow your own pursuits in your free time at school, no one will mind. If that's what you want, then it's a great scenario for you.
However, some people might find the alienation to be a bit down-heartening. If that's you, then you can use your time to create resources or displays that make you a more prominent part of the school. If you do this, the fact that 90% of ALTs don't may work in your favour enough to impress your coworkers/employer.
In short, if you can take pride in this job which literally anyone can do moderately, and do it well, it won't be as easy as it has a reputation for being. Or, you could just slack off and enjoy it. "Both of OK".
But the truth is that Japan is a very difficult place for a foreigner to find permanent work. If you're working the ALT gig and thinking about a long-term stay in Japan, then you had better take every opportunity you can to make yourself more employable.
Being an ALT series:
Part #2: Your Role
Part #3: A Typical Day
Part #4: Planning Classes
Part #5: Getting Drunk on Duty (the Enkai)
Part #6: 4 Japanese Teachers You May Have to Work With
Part #7: Physical Resources
Part #8: Making Activities
Part #9: Hopping Between Schools
Part #10: More School Hopping
Part #11: Getting Molested At School
Part #12: It Is Really This Easy?
Part #13: Demo Lessons
Part #14: Brushing Your Teeth At School
Part #15: Elementary Lesson Plan Example
Special: Being an ALT Documentary
Part #16: School Lunch
Part #17: What to Wear
Part #18: What to Keep in Your Desk