Congratulations on becoming the owner of a slightly-used (though still very capable) ALT (Assistant Language Teacher). A few guidelines to ensure that your ALT stays happy, healthy, and won't experience an emotional breakdown in the middle of use.
1) Be sure to give your ALT plenty of water.
* Chances are your ALT is from a much drier climate and not used to these hot and humid summers in Japan. Be sure to sprinkle them with water from time to time to ensure proper hydration.
2) Occasionally interpret what the other teachers/faculty/staff are saying around the ALT.
* Even if we're diligently studying Japanese on our own, the fact of the matter is we'll never be native speakers. It can be really intimidating having a bunch of people talking around and about you with no idea of what's being said.
3) Let your ALT know what s/he can do better in the class.
* You spend more time with your students and are therefore much more in touch with their needs. If the ALT isn't really connecting with the students, help them out. It will be much appreciated!
4) Don't forget to feed your ALT!
* Even though your ALT comes from a far off land, they're really no different from you! They also need plenty of nutrients in order to function properly. And much like you, they get these nutrients from eating. It's true, many ALTs even like the same foods that you like. Try giving them some nori or even some natto. You'll be surprised what a hungry gaijin will eat!
5) Lastly, be sure to let your ALT know when it's time to go home.
* Many ALTs are unsure or even unaware of cultural norms when it comes to leaving school. Though they are teaching assistants they aren't regular teachers and so they don't work the same hours. You need to let your ALT know it's OK for them to return to their domicile to rest before their next day teaching. If you don't, they may end up never leaving, and you don't want to find a dead ALT in the broom closet 6 months from now do you? I didn't think so...
If you follow these steps, you'll have a happy and genki ALT for a long time to come. I guarantee it.