2 cups baking soda
2 cups white vinegar
4 cups water
Put the baking soda in a plastic container and slowly add the vinegar. I do mean slowly...remember the volcano that you made in 5th grade science class...that happened by mixing baking soda and vinegar.
Next just add water and mix (I just used a gallon container with a lid and just shook it up).
Use 1/4 cup per load during final rinse cycle (or 1/3 cup if you have hard water).