Humane way to stop dog barking?

My older dog has become quite a brat! Whenever we leave the house, he barks very loudly until he tires himself out. I know that this has to do with seperation anxiety, but this is very unacceptable; we live in an apartment and his barking resonates quite loudly through the building. Does anyone know of any way to prevent the dog from barking when we leave the house?