Buy some trisodium phosphate at a hardware store and add a half a teaspoon or so to each wash.
http://www.castlewholesalers.com/SAVOGRAN-10621-1Lb-TSP-Trisodium-Phosphate.html
Stock up now, my local home despot store carries Tri-sodium Phosphate that (in the fine print) says it contains no phosphates.