To get a good grip on a regrout, you need to dig out at least 1/2 of the depth of the grout. Good ventillation and good housekeeping practices can minimize two of the three things for mildew: moisture, food, spores. You can't do anything about the spores, but if you can keep down the other two, you can keep mildew (or any mold) from growing. Using a fan, leaving the door or shower curtain open or wiping the walls down after use can limit moisture. Cleaning periodically can keep down the body oil, conditioner, soap scum, etc. that feeds the mold, but the biggest thing is the moisture.