Create a journal or database of your characters! This is a great tool to have if you can do several voices. Give each of your characters names, ages, birthdates, favorite colors, favorite foods, species (they don't have to be human), and any other details that make them special and memorable for you.
Now, if all you have is one voice, the only voice you use everyday, still do the same thing. There are several voice actors that can only do one voice and still make quite an impressive living. Anyone ever heard of Yeardly Smith or Patrick Warburton???