Evacuations will likely occur after an earthquake when fires are spreading or buildings are in danger of being destroyed by landslides, etc. City police and fire authorities will issue evacuation advice. Americans, as well as others affected by the disaster, will need to seek assistance from the Japanese authorities.
To prepare, take a walking pre-survey of the designated place of evacuation nearest your home and office. Also be familiar with the location of the ward office and the telephone number of the Disaster Relief Headquarters for your ward. The Tokyo Metropolitan Government has maps of evacuation points. You should phone your ward office and ask which evacuation points are nearest your home and office. Try to get detailed maps (with landmarks/street names in kanji as well) of those locations.
Even if your building is standing after the quake and you are not asked to evacuate, the evacuation points will have fresh water, food and medical supplies.