Turn down the temp on the hot water heater. In summer in Nagoya you do not need hot water anyway!
Wrap your water heater in an insulating jacket. I used one of those foam mats you sit on at festivals; it cost me 500 yen.
Try to use a fan instead of an air conditioner.
Don’t try to cool the whole house; shut off the room you are using and run the air conditioner only in there. (If you must use it!)
Set the air conditioner to keep the house a little warmer this year, Japanese offices will generally keep the temperature at 28 degrees C. If you need to go colder, try doing so incrementally instead of going immediately down to 20! Every little bit helps.
Clean out the filter on your air conditioner. This will lessen the load on the unit, making it run more efficiently.
Buy energy-efficient compact fluorescent bulbs
Install low-flow shower heads to reduce your hot water use.
When you shut down your computer, shut down your monitor as well. Many people just let it run until they start up the computer again.