No juice on the battery and you don't know how to unlock the doors?

Go to the passenger side door, insert the key, pull the handle and turn the key towards the unlock position. Keep turning past the normal position. There may be some resistance, but it will open.

When locking the car with no power on, for example if you are making a major repair on the car and want to lock the doors for the night, simply lock all the other doors from inside the car. Then get out and pull the handle on the passengers side door and depress the lock on the inside of the door. You can now close the door. Smack! The door is closed and locked. Did you remember your key, or is it still in the ignition? :-) If you are a bit more cautious person, you can just lock the last door with the key from the outside, but where's the excitement in that?

You can get into the engine bay, if the power fails and the above solution create problems. It is not as sophisticated as the key solution, but it will work anyway. You can emulate the action of pulling the hood release lever inside the car, by using a screwdriver. Insert the screwdriver between the main hood hinge and the arm with the release cable attached and pull the arm towards the side of the car, supporting the base of the screwdriver on the main hood hinge. A picture is worth a thousand words: