When I have a new gi that I want to break in and soften, I put in about 1 - 2 cups of white vinegar to the wash cycle. I repeat the agitation cycle several times and let it soak for a few hours.
For regular maintenance, the white vinegar is typically added to the rinse cycle. About 1/2 cup.
One last question and then I swear I'll leave you alone. When you break in a gi with the vinegar on hand; do you wash it in warm or hot water as part of that break in process. Will water temp alter the way the gi comes out in the end with the vinegar added to the mix?
I have one gi that has shrunk a lot with warm water and hot laundry dry. Now I only wash it in cold water and hang dry. I just ordered a new gi and I'm a little hesitant to "break it in" for fear of shrinkage.