Your planning will be affected by how long you plan to be gone. Some emergencies are temporary – such as a hurricane or a chemical spill – and you can return in a few days or hours, as soon as emergency responders let you. A worst-case scenario that might require long-term evacuation or permanent relocation would include a forest fire that results in the loss of your home, or a nuclear event that leaves unsafe levels of radiation in the area, like much of the country around Chernobyl.
For many emergencies, you may not know how long you will be gone. I am sure all of the Katrina victims who evacuated thought they would return, but many never did. Or in a wild fire, you do not know if your house will survive, allowing a rapid return, or be destroyed, requiring months to rebuild.
For a short term emergency, you can live out of your bug-out-bag (see section below). For potentially long term evacuations you will want to take more, while for permanent evacuations, you will want to take as much as possible.
In the latter scenarios, irreplaceable objects of value (financial or personal) all should be saved, as should important paperwork such as birth and marriage certificates, insurance policies, stocks and bonds and family photos. In this computerized age, you may well want to rescue your computer, so you can continue to access critical online information and accounts. At the least, pull out your hard drive and bring it along.