Buy a box of Oxi-Clean Stain Remover, comes in an 8.5 pound box as powder. Toss in a tablespoon (or less) into every wash load. It puts the phosphates back into the soap mix and clothes come out sparkling clean. Last forever. Keep the box tightly closed when not in use, or the powder will tend to cake at the top of the container. If it cakes up, chop at it with a screwdriver or other implement. Oxi-Clean is much safer to handle than TSP.
$4 at Home Depot. http://www.homedepot.com/p/SAVOGRAN-1-lb-Box-TSP-Heavy-Duty-Cleaner-10621/202935861